Abstract
Objective
To investigate the effect of inactivated coronavirus disease 2019 (COVID-19) vaccination on frozen-thawed embryo transfer (FET) outcomes.
Methods
This retrospective cohort study enrolled 1,210 patients undergoing FET cycles in a single university-affiliated hospital between July 1, 2021, and May 1, 2022. Of them, 387 women with two full doses of inactivated SARS-CoV-2 vaccines (CoronaVac or BBIBP-CorV) after oocyte retrieval were assigned to the vaccinated group, while 823 were unvaccinated as controls. Propensity score matching and multiple regression analysis were applied to control for baseline and cycle characteristics (19 covariates in total).
Results
There were 265 patients in each group after matching. The rates of clinical pregnancy (58.5% vs. 60.8%; P = 0.595) and live birth (44.4% vs. 48.8%; P = 0.693) were similar between vaccinated and unvaccinated patients, with adjusted odds ratios of 0.89 (95% confidence interval [CI] 0.61–1.29) and 1.31 (95% CI 0.37–4.56), respectively. Consistently, no significant differences were found in serum human chorionic gonadotropin levels as well as biochemical pregnancy, biochemical pregnancy loss, and embryo implantation rates. Based on the time interval from vaccination to FET, vaccinated patients were further subdivided into two categories of ≤2 months and >2 months, and the outcomes remained comparable.
Conclusion
Our study showed that inactivated COVID-19 vaccination in women did not have measurable detrimental impact on implantation performance and live birth outcome during FET treatment cycles. This finding denies the impairment of endometrial receptivity and trophoblast function by vaccine-induced antibodies at the clinical level.
Keywords: COVID-19, SARS-CoV-2, Vaccination, Frozen-thawed embryo transfer, Live birth
1. Introduction
Since the first case was discovered in December 2019, coronavirus disease 2019 (COVID-19) has spread rapidly and become a global pandemic. To date, over 560 million confirmed cases have been reported worldwide and more than 5 million in China [1]. Among women of reproductive age, pregnant women are more susceptible to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, with increased risks of adverse outcomes including premature delivery and stillbirth [2], [3].
To effectively reduce morbidity and mortality from the pandemic, a number of vaccines have been developed in an urgent rush. Consistent with other countries, China has launched a large-scale vaccination campaign against COVID-19, which includes inactivated vaccines, adenovirus vector vaccines and recombinant subunit vaccines [4]. As of July 20, 2022, the total number of COVID-19 vaccine doses in China exceeded 3.4 billion, with inactivated vaccines accounting for the largest proportion [4], [5], [6]. However, the promotion of vaccines has been hampered by widespread concerns regarding their possible detrimental effect on reproductive health. In a survey of unvaccinated individuals, 23.9% were convinced that there were negative fertility impacts, and 21.8% thought there may be [7]. The initial argument is that the placental syncytin-1 protein is similar to the spike protein of SARS-CoV-2 and therefore, vaccine-induced antibodies may result in cross-reactivity and infertility [8], [9]. Despite the lack of experimental evidence, such conjectures persist on various social media platforms and undermine vaccination coverage.
In an effort to address this issue, emerging cohort studies have been performed and concluded that COVID-19 vaccination had no adverse effects on ovarian reserve, oocyte quality, as well as embryonic development during in vitro fertilization (IVF) treatment [6], [10], [11], [12], [13], [14]. By taking a specific focus on frozen-thawed embryo transfer (FET), four studies also demonstrated a neutral influence of mRNA vaccines on endometrial receptivity and embryo implantation [15], [16], [17], [18]. Nonetheless, the association of inactivated COVID-19 vaccines with FET outcomes has been unclear and remains clarification. In addition, none of these studies reported the key outcome of live birth rate, indicating the need of longer follow-up for corroboration.
The aim of this study was to comprehensively evaluate the effect of inactivated SARS-CoV-2 vaccination on pregnancy outcomes in a large cohort of 1210 FET patients.
2. Materials and methods
2.1. Study design and participants
This was a single-center retrospective cohort study, in which all patients were recruited from the Center for Reproductive Medicine, Jiangxi Maternal and Child Health Hospital between July 1, 2021, and May 1, 2022. All procedures performed in this work were in accordance with the standards of Reproductive Medicine Ethics Committee of Jiangxi Maternal and Child Health Hospital (approval No. 2022-03) and all patient information were used anonymously.
Prior to the initiation of FET, participants were screened for COVID-19 using epidemiological questionnaire, temperature measurement, and pharyngeal swab polymerase chain reaction test for SARS-CoV-2 RNA. Vaccination status, including vaccine type, date, dose, and manufacturer, were also obtained and verified via access to immunization records. Patients were assigned to the vaccinated group if they had completed two doses of inactivated COVID-19 vaccine (CoronaVac or BBIBP-CorV) and to the control group if they had not been vaccinated (Figure S1). To eliminate the confounding effect of vaccination on gamete and embryo, we excluded cycles with paternal vaccination before ovarian stimulation from analysis. Other exclusion criteria were: 1) history of SARS-CoV-2 infection by self-report; 2) receipt of other types of SARS-CoV-2 vaccines, including recombinant vaccine (ZF2001) and adenovirus type-5 vector-based vaccine (CanSino) [19]; 3) partial vaccination before FET; 4) donor sperm or oocyte cycle; 5) preimplantation genetic testing cycle; 6) second or higher cycle in the same patient during the timeframe; and 7) lost to follow-up or missing data in the electronic health record.
2.2. Endometrial preparation and embryo transfer
The implementation of three different endometrial preparation protocols was jointly determined by patient preference and physician discretion. During the natural cycle, follicular development was closely monitored by transvaginal ultrasonography and blood sampling, starting from day 10 of the menstrual cycle and every one or two days thereafter. In occurrence of ovulation, intramuscular injection of progesterone (40 mg/d; Xianju Pharma, China) was initiated for endometrial transformation, followed by cleavage-stage embryo transfer on the fourth day or blastocyst transfer on the sixth day.
In the artificial cycle, oral estradiol valerate (4–6 mg/d; Progynova, Bayer, Germany) was given from the second or third day of menstruation. After 7 days, the absence of dominant follicle was confirmed by ultrasound, while the estradiol dose was adjusted according to endometrial thickness. When oral estradiol was used for 12–14 days with the thickness reaching 7 mm, intramuscular progesterone (80 mg/d; Xianju Pharma, China) was administered daily and FET was scheduled for cleavage-stage embryos after 4 days or blastocysts after 6 days. For patients with endometriosis, adenomyosis, hyperandrogenic polycystic ovary syndrome and history of cesarean section, a long-acting gonadotropin-releasing hormone (GnRH) agonist of 3.75 mg leuprorelin acetate (Lizhu Pharmaceutical Trading Co, China) was given on day 2 or 3 of the preceding menstrual cycle. On 28 days post-injection, patients were required to return to the hospital for the same procedure as the artificial cycle described above.
With the assistance of abdominal ultrasound, up to two embryos per patient were transferred in one FET cycle. After thawing, top-quality embryos on day 3 were defined as those with 7–10 cells and grade I–II based on the Cummins’s morphological criteria. Blastocysts were graded according to the Gardner and Schoolcraft system, and the top-quality blastocysts were those with an expansion score ≥4, an inner cell mass ≥B, and a trophectoderm score ≥B. Luteal support was carried out from the day of transfer via oral (20 mg/d; Duphaston, Abbott Biologicals, USA) and vaginal (90 mg/d; Crinone, Merck Serono, Switzerland) routes, and maintained until 10 weeks of gestation when a pregnancy was established.
2.3. Outcome measures
The primary outcome was the rate of clinical pregnancy per cycle, defined as the presence of one or more gestational sacs detected under transvaginal ultrasound one month after embryo transfer. Secondary outcomes included biochemical pregnancy, biochemical pregnancy loss, embryo implantation, and live birth rates. Biochemical pregnancy was defined as a serum β-human chorionic gonadotropin (hCG) level exceeding 5 IU/L on day 12 after cleavage-stage embryo transfer or day 10 after blastocyst transfer. Biochemical pregnancy loss was defined as the loss of hCG positivity prior to clinical pregnancy in patients with biochemical pregnancy. The implantation rate was calculated as the ratio between the number of gestational sacs detected and the number of embryos transferred. Live birth was defined as a viable infant delivered after a complete gestational period of 24 weeks or more, and was available for 318 women with embryo transfer before October 3, 2021.
2.4. Sample size calculation
Sample size calculation was performed with an online cost-free power and sample size calculator at https://powerandsamplesize.com/ (HyLown Consulting LLC, USA). The category chosen was “Compare 2 Proportions: 2-Sample, 2-Sided Equality”. According to prior data from our center, the primary outcome of clinical pregnancy rate per FET cycle was 60%. Assuming α = 0.05 and 80% power, it was calculated that 384 patients were required for each group to detect an absolute difference of 10% between vaccinated and unvaccinated patients.
2.5. Statistical analyses
In this study, the SAS version 9.4 (SAS Institute Inc., USA) was employed for all statistical analyses. The Shapiro-Wilk test was applied to evaluate the normality of quantitative data, with values presented as means ± standard deviations. Student's t-test or Mann-Whitney U test was performed to compare differences between groups as appropriate. Categorical variables were summarized as numbers and percentages, and analyzed using χ2 test or Fisher's exact test. All tests were two-tailed and P < 0.05 was considered statistically significant.
Propensity score matching (PSM) was applied to balance the baseline and cycle characteristics of the vaccinated group with those of the control group at a ratio of 1:1 via the nearest-neighbor matching algorithm. A caliper width of 0.2 standard deviations was determined. The following variables were selected as potential confounders for matching, including maternal age at retrieval, maternal age at transfer, body mass index, infertility type (primary or secondary), infertility duration, infertility diseases (tubal factor, male factor, ovulatory dysfunction, diminished ovarian reserve, endometriosis, or uterine factor), ovarian stimulation protocol (agonist, antagonist, or others), fertilization method (IVF or intracytoplasmic sperm injection), prior embryo transfer failure (0, 1–2, or ≥3), endometrial preparation regimen (natural cycle, artificial cycle, or artificial cycle with GnRH agonist), endometrial thickness, number of embryos transferred (1 or 2), embryo developmental stage (cleavage or blastocyst), and whether top-quality embryo was transferred (yes or no).
To further assess the independent association between inactivated SARS-CoV-2 vaccination and FET pregnancy outcomes, multiple logistic regression analyses were performed using both pre- and post-matching data. Adjusted odds ratios (aORs) with 95% confidence interval (CIs) were computed after controlling for the same covariates in PSM. For subgroup analysis, the time interval between the last vaccine dose and embryo transfer was classified as ≤2 and >2 months in accordance with the guideline of the European Society of Human Reproduction and Embryology (ESHRE) [20].
3. Results
Initially, a total of 2,251 FET cycles were screened for eligibility and 1,210 patients were included for the final analysis. Depending on the status of inactivated SARS-CoV-2 vaccination, the participants were further divided into the vaccinated group (n = 387) and control group (n = 823). The detailed study flowchart is depicted in Figure S2.
Table 1 presents the baseline demographics and cycle characteristics before (left) and after (right) matching. Prior to PSM processing, the two groups differed significantly in maternal age at retrieval and transfer, infertility duration and type, proportion of diminished ovarian reserve and endometriosis, ovarian stimulation and endometrial preparation protocol, as well as number, stage, and quality of transferred embryos. After subsequent PSM, 265 individuals remained in each group, and all parameters were comparably adjusted. The distribution of propensity scores before and after PSM are demonstrated in Figure S3.
Table 1.
Baseline and cycle characteristics grouped by the vaccination status.
| Before matching |
After matching |
|||||
|---|---|---|---|---|---|---|
| Vaccinated (n = 387) |
Unvaccinated (n = 823) |
P value | Vaccinated (n = 265) |
Unvaccinated (n = 265) |
P value | |
| Age at OPU (years) | 30.3 ± 4.7 | 31.3 ± 5.4 | 0.002 | 31.0 ± 4.8 | 30.9 ± 4.7 | 0.824 |
| Age at ET (years) | 32.8 ± 4.5 | 32.1 ± 5.4 | 0.010 | 32.6 ± 4.7 | 32.3 ± 4.8 | 0.557 |
| Body mass index (kg/m2) | 22.1 ± 2.5 | 22.0 ± 3.2 | 0.256 | 22.2 ± 2.6 | 22.2 ± 3.3 | 0.474 |
| Infertility duration (years) | 6.3 ± 3.5 | 4.8 ± 3.4 | <0.001 | 5.4 ± 3.2 | 5.0 ± 3.1 | 0.056 |
| Infertility type, n (%) | <0.001 | 0.489 | ||||
| Primary | 72 (18.6) | 295 (35.8) | 66 (24.9) | 73 (27.6) | ||
| Secondary | 315 (81.4) | 528 (64.2) | 199 (75.1) | 192 (72.5) | ||
| Infertility diseases | ||||||
| Tubal factor, n (%) | 266 (68.7) | 588 (71.5) | 0.334 | 179 (67.6) | 198 (74.7) | 0.069 |
| Male factor, n (%) | 98 (25.3) | 199 (24.2) | 0.667 | 52 (19.6) | 68 (25.7) | 0.097 |
| Ovulatory dysfunction, n (%) | 64 (16.5) | 156 (19.0) | 0.309 | 41 (15.5) | 40 (15.1) | 0.904 |
| Diminished ovarian reserve, n (%) | 47 (12.1) | 157 (19.1) | 0.003 | 41 (15.5) | 38 (14.3) | 0.714 |
| Endometriosis, n (%) | 35 (9.0) | 43 (5.2) | 0.012 | 24 (9.1) | 20 (7.6) | 0.529 |
| Uterine factor, n (%) | 47 (12.1) | 84 (10.2) | 0.312 | 35 (13.2) | 28 (10.6) | 0.348 |
| Ovarian stimulation protocol, n (%) | <0.001 | 0.493 | ||||
| GnRH agonist | 316 (81.7) | 548 (66.6) | 201 (75.9) | 211 (79.6) | ||
| GnRH antagonist | 33 (8.5) | 136 (16.5) | 31 (11.7) | 29 (10.9) | ||
| Others | 38 (9.8) | 139 (16.9) | 33 (12.5) | 25 (9.4) | ||
| Fertilization method, n (%) | 0.205 | 0.760 | ||||
| IVF | 299 (77.3) | 608 (73.9) | 203 (76.6) | 200 (75.5) | ||
| ICSI | 88 (22.7) | 215 (26.1) | 62 (23.4) | 65 (24.5) | ||
| Prior ET failure, n (%) | 0.525 | 0.797 | ||||
| 0 | 212 (54.8) | 477 (58.0) | 132 (49.8) | 131 (49.4) | ||
| 1–2 | 167 (43.2) | 327 (39.7) | 128 (48.3) | 131 (49.4) | ||
| ≥3 | 8 (2.1) | 19 (2.3) | 5 (1.9) | 3 (1.1) | ||
| Endometrial preparation, n (%) | 0.011 | 0.649 | ||||
| Artificial cycle with GnRH agonist | 215 (55.6) | 531 (64.5) | 150 (56.6) | 160 (60.4) | ||
| Artificial cycle | 130 (33.6) | 224 (27.2) | 89 (33.6) | 83 (31.3) | ||
| Natural cycle | 42 (10.9) | 68 (8.3) | 26 (9.8) | 22 (8.3) | ||
| Endometrial thickness (mm) | 9.3 ± 1.7 | 9.5 ± 1.8 | 0.477 | 9.3 ± 1.6 | 9.3 ± 1.8 | 0.521 |
| No. of embryos transferred, n (%) | <0.001 | 0.223 | ||||
| 1 | 199 (51.4) | 318 (38.6) | 130 (49.1) | 116 (43.8) | ||
| 2 | 188 (48.6) | 505 (61.4) | 135 (50.9) | 149 (56.2) | ||
| Embryo developmental stage, n (%) | <0.001 | 0.927 | ||||
| Cleavage | 121 (31.3) | 371 (45.1) | 89 (33.6) | 88 (33.2) | ||
| Blastocyst | 266 (68.7) | 452 (54.9) | 176 (66.4) | 177 (66.8) | ||
| Transfer of ≥1 top-quality embryo, n (%) | 206 (53.2) | 522 (63.4) | 0.001 | 145 (54.7) | 148 (55.9) | 0.793 |
Notes: Data are presented as mean ± standard deviation or number (percentage).
Abbreviations: OPU, oocyte pick-up; ET, embryo transfer; IVF, in vitro fertilization; ICSI, intracytoplasmic sperm injection; GnRH, gonadotropin-releasing hormone.
Pregnancy outcomes grouped by the vaccination status are shown in Table 2 . After matching, the rates of clinical pregnancy (58.5% vs. 60.8%; P = 0.595) and live birth (44.4% vs. 48.8%; P = 0.693) were similar between vaccinated and unvaccinated patients. Consistently, there were no statistically significant differences in biochemical pregnancy, biochemical pregnancy loss, and embryo implantation rates. In addition to pregnancy outcomes, serum β-hCG levels were also compared among patients with biochemical pregnancy after cleavage-stage embryo or blastocyst transfer (Fig. 1 ). The lack of a discernible difference before and after PSM suggested that inactivated COVID-19 vaccines had no measurable impact on hCG production.
Table 2.
Pregnancy outcomes grouped by the vaccination status.
| Before matching |
After matching |
|||||
|---|---|---|---|---|---|---|
| Vaccinated (n = 387) | Unvaccinated (n = 823) | P value | Vaccinated (n = 265) | Unvaccinated (n = 265) | P value | |
| Biochemical pregnancy, n (%) | 285 (73.6) | 625 (75.9) | 0.388 | 191 (72.1) | 200 (75.5) | 0.374 |
| Biochemical pregnancy loss, n/N (%) | 51/285 (17.9) | 126/625 (20.2) | 0.423 | 36/191 (18.9) | 39/200 (19.5) | 0.870 |
| Clinical pregnancy, n (%) | 234 (60.5) | 499 (60.6) | 0.956 | 155 (58.5) | 161 (60.8) | 0.595 |
| Embryo implantation, n/N (%) | 274/575 (47.7) | 610/1328 (45.9) | 0.490 | 186/400 (46.5) | 192/414 (46.4) | 0.972 |
| Live birth a, n/N (%) | 17/37 (46.0) | 151/281 (53.7) | 0.372 | 12/27 (44.4) | 41/84 (48.8) | 0.693 |
Notes: Data are presented as number (percentage).
Live birth outcomes were completely followed-up for 318 women with embryo transfer before October 3, 2021.
Fig. 1.
Comparison of serum human chorionic gonadotropin (hCG) levels in vaccinated and unvaccinated patients with biochemical pregnancy (A) before and (B) after matching. The measurement of hCG was performed on day 12 after cleavage-stage embryo transfer or day 10 after blastocyst transfer. Abbreviation: NA, not available, as the unvaccinated group included only one patient with single cleavage-stage embryo transfer.
On further adjusted analysis, there was still no significant association between female vaccination and the odds of clinical pregnancy (aOR 0.89, 95% CI 0.61–1.29) or any of the secondary outcomes assessed: biochemical pregnancy (aOR 0.82, 95% CI 0.55–1.25), biochemical pregnancy loss (aOR 1.08, 95% CI 0.62–1.89), or live birth (aOR 1.31, 95% CI 0.37–4.56) (Table 3 ). Multivariable logistic regression was also performed on the full cohort before matching, with no associations observed in all four outcomes.
Table 3.
Association between vaccination and pregnancy outcomes on crude and adjusted analysis.
| Before matching |
After matching |
||||
|---|---|---|---|---|---|
| cOR (95% CI) | aOR (95% CI) a | cOR (95% CI) | aOR (95% CI) a | ||
| Biochemical pregnancy | 0.89 (0.67–1.17) | 0.74 (0.53–1.03) | 0.84 (0.57–1.24) | 0.82 (0.55–1.25) | |
| Biochemical pregnancy loss | 0.86 (0.60–1.24) | 1.01 (0.66–1.55) | 0.96 (0.58–1.59) | 1.08 (0.62–1.89) | |
| Clinical pregnancy | 0.99 (0.78–1.27) | 0.84 (0.63–1.14) | 0.91 (0.64–1.29) | 0.89 (0.61–1.29) | |
| Live birthb | 0.73 (0.37–1.46) | 0.68 (0.29–1.62) | 0.84 (0.35–2.01) | 1.31 (0.37–4.56) | |
Abbreviations: cOR, crude odds ratio; aOR, adjusted odds ratio; CI, confidence interval.
Analyses were adjusted for age at retrieval, age at transfer, body mass index, type of infertility, infertility duration, infertility diseases, ovarian stimulation protocol, fertilization method, prior embryo transfer failure, endometrial preparation regimen, endometrial thickness, number of embryos transferred, embryo developmental stage, and embryo quality.
Odds ratios for live birth were calculated on the basis of 318 and 111 women before and after matching, respectively.
Based on the vaccination interval to embryo transfer, vaccinated participants were further divided into two categories of ≤2 months and >2 months. As displayed in Table 4 and Figure S4, comparison between the subgroups revealed similar pregnancy outcomes as well as early serum β-hCG levels.
Table 4.
Comparison of pregnancy outcomes based on the vaccination interval to embryo transfer.
| ≤2 months (n = 59) | >2 months (n = 328) | P value | cOR (95% CI) | aOR (95% CI) a | |
|---|---|---|---|---|---|
| Biochemical pregnancy, n (%) | 44 (74.6) | 241 (73.5) | 0.860 | 1.06 (0.56–2.00) | 1.66 (0.77–3.58) |
| Biochemical pregnancy loss, n/N (%) | 10/44 (22.7) | 41/241 (17.0) | 0.363 | 1.44 (0.66–3.13) | 1.53 (0.60–3.93) |
| Clinical pregnancy, n (%) | 34 (57.6) | 200 (61.0) | 0.628 | 0.87 (0.50–1.53) | 1.15 (0.58–2.27) |
Notes: Data are presented as number (percentage).
Abbreviations: cOR, crude odds ratio; aOR, adjusted odds ratio; CI, confidence interval.
Analyses were adjusted for age at retrieval, age at transfer, body mass index, type of infertility, infertility duration, infertility diseases, ovarian stimulation protocol, fertilization method, prior embryo transfer failure, endometrial preparation regimen, endometrial thickness, number of embryos transferred, embryo developmental stage, and embryo quality.
4. Discussion
In this retrospective cohort study, the analysis of 1,210 patients revealed that inactivated SARS-CoV-2 vaccination after oocyte retrieval had no detrimental impact on subsequent FET outcomes. Furthermore, the results demonstrated that pregnancy rates were not significantly affected by the time interval between vaccination and embryo transfer. This updated evidence can be helpful for clinicians to promote vaccination coverage in unvaccinated patients, to reduce psychological burden of vaccinated patients, and to arrange FET cycles at their earliest convenience.
A number of studies have focused on the effects of COVID-19 vaccination on ovarian stimulation during IVF treatment, and found no significant association with oocyte yield or embryo quality [6], [10], [11], [12], [13], [14]. Consistently, three studies reported that the perlecan level as well as hormonal, lipid and metabolic profiles in follicular fluid were comparable between vaccinated and unvaccinated participants, suggesting the unaltered growth and development microenvironment of follicles [21], [22], [23]. In terms of pregnancy outcomes after fresh embryo transfer, Avraham et al. [10] enrolled 200 vaccinated and 200 age-matched unvaccinated patients, demonstrating no adverse impact of BNT162b2 on clinical pregnancy rate (32.8% vs. 33.1%, P = 0.96). In another study by Jacobs et al. [11], 142 vaccinated (mRNA or adenovirus vector) women were compared with 138 unvaccinated women. Using the logistic regression model adjusted for age and body mass index, it also revealed no significant difference in ongoing pregnancy (aOR 0.79, 95% CI 0.48–1.29). Similar outcomes have been observed concerning inactivated vaccines [6], [13], [14], which imply their safety in female reproduction as other vaccine types among fresh IVF cycles.
By separating the procedures of ovarian stimulation and embryo transfer, FET offers an excellent model to evaluate the independent influence of infection and/or vaccination on endometrial receptivity for embryo implantation. Prior to our study, there has been a paucity of literatures addressing the impact of COVID-19 itself on FET outcomes. The first study by Morris et al. [18] included 143 women undergoing their first single FET, and found that seropositivity for the SARS-CoV-2 spike protein due to prior infection or vaccination did not alter implantation rates compared with seronegative women. Similarly, by retrospectively analyzing 672 FET cycles, Aizer et al. [16] demonstrated no significant differences in biochemical, clinical and ongoing pregnancy rates among infected, vaccinated and control patients. In contrast, Youngster et al. [10] showed that past SARS-CoV-2 infection was associated with decreased odds of clinical pregnancy (aOR 0.325, 95% CI 0.106–0.998; P = 0.05), which was more evident in the subgroup of women transferred within 60 days after infection (OR 0.072, 95% CI 0.012–0.450; P = 0.005). Mechanistically, a recent study found no SARS-CoV-2 RNA in the infected endometrial tissue, suggesting that COVID-19 may not directly invade the female reproductive tract [24]. Instead, transcriptomic profiling indicated altered endometrial gene expression in 75% of women, which were mainly enriched in immunological, inflammatory, and metabolic processes [24]. These indirect changes may thus pose potential detrimental effects on endometrial receptivity and consequently FET outcomes. However, current studies are still limited by small sample sizes of infected patients (ranging from 20 to 41), and larger cohorts are warranted for a more decisive conclusion.
To our knowledge, only several prior studies have addressed FET outcomes following COVID-19 vaccination. Aharon et al. included 947 patients undergoing single euploid FET, and the adjusted analysis demonstrated no association between vaccination and clinical pregnancy (aOR 0.79, 95% CI 0.54–1.16) or any other pregnancy outcomes [15]. More recently, Brandão et al. [17] compared cycle outcomes in women who underwent euploid FET one year before the pandemic (n = 3272) with those who had received at least one dose of BNT162b2 or mRNA-1273 (n = 890). Based on the large caseloads, they concluded that injection of vaccines against COVID-19 had no measurable effect on clinical pregnancy and sustained implantation rates, regardless of the number of doses and the time interval from vaccination to transfer. Similar findings have been observed in the other two studies [16], [18], while all studies included only mRNA vaccines, which should not be directly extrapolated to other types of vaccine. In addition, these studies did not differentiate the vaccination timepoints before or after oocyte retrieval to rule out its confounding effects on gametes and embryos. In this regard, our study was conducted and clearly proved that inactivated SARS-CoV-2 vaccines did not affect the treatment outcomes of FET.
According to a meta-analysis conducted in 2021, there was a declining tendency in the willingness to be vaccinated across countries when COVID-19 vaccines became available [25]. Women showed lower intentions to be vaccinated than other populations, especially among those who were currently or planning to become pregnant. Such hesitancy has been thought to be derived from fertility concerns [7], and exaggerated by the widespread dissemination of unproven claims. One of them is the similarity between the SARS-CoV-2 spike protein and the human placental protein syncytin-1. Syncytin-1 is encoded by the human endogenous retrovirus W (HERV-W) gene in trophoblast cells and plays an important role in trophoblast fusion during placenta formation [26]. Nonetheless, recent studies have shown limited homology and there is no cross-reactivity of antibody against SARS-CoV-2 spike protein to syncytin-1 [8], [9], [27]. Moreover, compared with unvaccinated adults, COVID-19 vaccinated participants did not present elevated levels of circulating anti-syncytin-1 antibodies by enzyme linked immunosorbent assay [28]. In our study, we found that inactivated vaccine administration had no significant effect on serum β-hCG levels during the earliest stage of pregnancy, which further denies the damage of trophoblast cells by vaccine-induced antibodies at the clinical level.
The consensus on the optimal time interval between vaccination completion and assisted reproductive treatment has been varied in different scientific societies. Due to the lack of evidence on the reproductive effects of COVID-19 vaccines, experts at the Beijing Human Assisted Reproductive Technology Center for Quality Control and Improvement recommend starting treatment after one month of vaccination for the stabilization of immune response [29]. Instead, the American Society for Reproductive Medicine recommends avoiding COVID-19 vaccination for at least three days only before and after a planned surgical procedure (e.g., oocyte retrieval) or outpatient treatment (e.g., embryo transfer) [30]. Holding a more cautious attitude, the ESHRE guideline suggests a two-month postponement to allow sufficient time for antibody development [20]. In the present study, we showed no significant difference in biochemical and clinical pregnancy rates between ≤2 and >2 months, suggesting a neutral impact of intervals between vaccination and embryo transfer. This finding is also supported by other studies using different intervals of 1, 1.8 or 3 months for categorization [6], [14], [17], which could be reassuring for women to prepare for FET promptly following vaccination.
To date, this is the first study to examine the effect of inactivated COVID-19 vaccines on FET live birth outcomes. One of the main strengths is that we assessed the rate of live birth, which is the key IVF outcome but has not been covered in previous studies due to limited follow-up duration. In addition, PSM was employed to control for confounding variables and minimize bias, which remains among the best approaches for drawing conclusions about causality from observational data [31]. The high consistency and reproductivity in multiple regression models further add to the robustness and reliability of our finding.
It should be emphasized that the current work also has some limitations to be resolved in the future. First, there were inherent bias and residual confounding associated with data from a retrospective cohort study [32]. For instance, although we considered embryo quality for adjustment, we did not screen euploid embryos for transfer, which ought to be verified in preimplantation genetic testing cycles. In addition, the type of inactivated vaccines (i.e. CoronaVac or BBIBP-CorV) was not recorded for subgroup analysis, whose potential confounding effect should be controlled in future prospective cohort studies. Second, the determination of COVID-19 history was based on self-report by patients without objective seropositivity test. In this regard, a misclassification risk may be present since the infected patients should be deemed immunized as those vaccinated. It also remains to be clarified whether vaccine-induced SARS-CoV-2 neutralizing antibody affects endometrial receptivity and embryo implantation in a concentration-dependent manner. Third, the study did not assess the status of newborns, which limits the determination of vaccine safety in the long-term period. Also, we did not complete the live birth follow-up of the entire cohort, resulting in a reduced sample size and potentially decreased statistical power. Finally, since this was a single-center cohort, the findings need to be corroborated by further multicenter studies for generalizability.
5. Conclusion
In conclusion, our work provides evidence that inactivated SARS-CoV-2 vaccination has no detrimental impact on endometrial receptivity and embryo implantation during FET treatment cycles. This finding further dispels the misconception that COVID-19 vaccines impair female fertility, which is reassuring for vaccinated women planning on pregnancy and could be informative for physicians in clinical counseling. Larger prospective cohort studies with continuous follow-up are needed to validate our conclusion.
6. Ethics approval and consent to participate
The study was approved by the Reproductive Medicine Ethics Committee of Jiangxi Maternal and Child Health Hospital (No. 2022-03). Informed contents were obtained from all patients for de-identified data use in scientific research.
7. Consent for publication
Not applicable.
8. Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Funding
The study was funded by the National Natural Science Foundation of China (82260315, 81960288) and Key Research and Development Program of Jiangxi Province (20203BBGL73159).
CRediT authorship contribution statement
Jialyu Huang: Conceptualization, Methodology, Investigation, Writing – original draft, Funding acquisition. Yiqi Liu: Formal analysis, Investigation, Writing – original draft, Visualization. Han Zeng: Formal analysis, Investigation, Data curation, Project administration. Lifeng Tian: Data curation, Writing – review & editing, Funding acquisition. Yina Hu: Data curation, Writing – review & editing. Jinxia He: Data curation, Writing – review & editing. Ling Nie: Data curation, Writing – review & editing. You Li: Data curation, Writing – review & editing. Zheng Fang: Formal analysis, Data curation. Weiping Deng: Data curation. Mengyi Chen: Data curation. Xia Zhao: Data curation. Dongxiang Ouyang: Data curation. Yuqing Fu: Data curation. Jiaying Lin: Conceptualization, Methodology, Writing – review & editing, Supervision. Leizhen Xia: Methodology, Formal analysis, Investigation, Visualization, Project administration. Qiongfang Wu: Conceptualization, Investigation, Writing – review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
None.
Footnotes
Supplementary data to this article can be found online at https://doi.org/10.1016/j.intimp.2022.109552.
Appendix A. Supplementary data
The following are the Supplementary data to this article:
Supplementary Fig. 1.
Supplementary Fig. 2.
Supplementary Fig. 3.
Supplementary Fig. 4.
Data availability
Data will be made available on request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Data will be made available on request.