Dear Editor,
We have read the recent article by Davide Moschese and Angelo Roberto Raccagni et al. published in this journal, which discusses monkeypox infection among individuals who were previously immunized with smallpox vaccination.1, 2 Although the number of confirmed monkeypox cases in China is currently low, the highly pathogenic and transmissible nature of the virus cannot be ignored. Studies have shown that Orthopoxvirus-specific IgG, a humoral immune response produced after vaccinia virus vaccination, has long-term persistence and can be effective in preventing monkeypox infection.3 However, the neutralizing antibodies that provide cross-protection may decrease over time, even though they can last for several years after vaccination. In this regard, we are interested in investigating the level of humoral response after 40 years of compulsory immunity in China, where a particular vaccinia virus vaccination strain called Tiantan was used. Furthermore, we are curious about the humoral response level in individuals who have never received Orthopoxvirus vaccination in China, where large-scale outbreaks of monkeypox have not been reported.
In China, the "Tiantan strain" attenuated smallpox vaccine was developed in early 1926 and subsequently became mandatory after 1950.4 With the success of the global mass vaccination campaign, smallpox was eradicated and declared as such by the World Health Organization in 1980, leading the Chinese government to cease smallpox vaccination in 1981.5, 6 Due to relaxed vaccination requirements during the 1970s,6 individuals born between 1972 and 1980 may not have been vaccinated. To differentiate between the vaccinated (pre-1971) and unvaccinated (post-1981) groups, the study excluded individuals born between 1972 and 1980. A total of 784 healthy volunteers, between the ages of 4 and 41 and 50–84 years old as of 2022, were recruited and divided into two birth cohorts based on their smallpox (Tiantan) vaccination history: pre-1971, the mandatory vaccination period, and post-1981. The monkeypox virus (MPXV) specific antibodies targeting A29L, A35R, and B6R antigens (homologous to A27L, A33R, and B5R in smallpox subunit vaccine) were measured using ELISA assays. These antigens were obtained from Sino Biology Company in China and the authors coated the ELISA plates with these antigens. The dilution level was determined through preliminary evaluations using box titration.
In this study, a subset of data presented in Fig. 1 confirmed our hypothesis that individuals born before 1971, who were presumably vaccinated against smallpox, had a significantly higher humoral response than those born after 1981, who showed minimal humoral response. We used ROC curves to establish the cut-off values for ELISA detection of specific antibodies against the MPXV and calculated the percentage of plasma samples positive for A35R, B6R, or both antigens, which are reported in Table 1. Surprisingly, only 38.2% of the subjects born before 1971 tested positive for either A35R or B6R, suggesting a low humoral response that may partially explain the increased incidence of monkeypox cases in patients who were previously vaccinated against smallpox,7, 8 given the critical role of humoral antibodies in protecting against monkeypox.3 Additionally, our findings indicate that A35R and B6R antigens may differentiate individuals who have been vaccinated against smallpox from those who have not. However, cross-immunity with A29L cannot be ruled out, although A35R and B6R exhibited better distinction than A29L in our study. The difference in response among A35R, B6R, and A29L may be due to A29L being expressed in E. coli while the other two antigens are HEK cell-expressed, although further validation is necessary. Information on the specific MPXV-specific antigens is presented in Supplemental Fig. 1.
Fig. 1.
The results of the enzyme-linked immunosorbent assay (ELISA). ELISA was used to determine MPXV-specific immunoglobulin G (IgG) levels, represented as optical density (OD) values at 450 nm (OD450). The data includes OD450 values (log2 transformed) for three targets, comparing two birth cohorts: the post-1981 and pre-1971 groups. The median value is indicated by the bar in the graph. Statistical significance, determined by the Wilcoxon test, is denoted by p ≤ 0.05.
Table 1.
Proportion of plasma samples positive to A35R, B6R or both antigens by ELISA.
| Birth yr | Post-1981 | Pre-1971 | p-value (chi-square test) |
|---|---|---|---|
| No. of subjects | 470 | 314 | |
| Cut-off Values | A35R 0.35 | B6R 0.42 | |
| A35R+B6R- | 2 (0.4%) | 28 (8.9%) | 2.065e-06 |
| A35R+B6R+ | 2 (0.4%) | 39 (12.4%) | 7.54e-09 |
| A35R-B6R- | 458 (97.5%) | 194 (61.8%) | <2.2e-16 |
| A35R-B6R+ | 8 (1.7%) | 53 (16.9%) | 8.329e-09 |
Despite its limitations, primarily related to sample representativeness, our study sheds light on the status of the Chinese population's immunity against MPXV Shenzhen, where our samples were collected, is a unique city that has undergone rapid urbanization and population growth, with an estimated 13 million residents in contrast to a population of just 30,000 in 1978.9 Thus, while our volunteers came from various regions in China, the sample size, though small, is diverse and representative. Our findings reveal that only a subset of the Chinese population born before 1971 retain specific IgG capable of reacting with A35R and B6R antigens of MPXV, with the post-1981 cohort exhibiting minimal specific IgG reaction. Although we lack information on the frequency of vaccination in the pre-1971 group, our results suggest that successful vaccination may confer long-lasting immunity.
Although it seems logical, there are few reports on surveys of humoral protection in healthy populations in low-risk areas, especially regarding the efficacy of different vaccines, including the Tiantan strain, which has affected one-fifth of the world's population. Our findings suggest that cross-immunity is partially antigen-specific, with approximately 38% of reactions related to a history of previous smallpox vaccination. Our data demonstrates that, without additional interventions, collective immunization status alone will not be sufficient to prevent MPXV transmission. Therefore, vaccination with the MPXV vaccine is necessary for at-risk populations, regardless of their previous smallpox vaccination history. China is now a low-risk country for MPXV epidemics, and we hope that our results will help prioritize anti-MPXV strategies and prevention in China. Particularly in the context of a renewed outbreak of MPXV last year and the limited vaccine availability worldwide.
Conflict of interest
We attest that there are no conflicts of interest related to the research or the manuscript.
Acknowledgments
This research was supported by Key scientific and technological project of Shenzhen Science and Technology Innovation Committee (KCXFZ202002011006190).
Footnotes
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jinf.2023.04.002.
Appendix A. Supplementary material
The information on the specific MPXV-specific antigens. Panel A shows the SDS-PAGE of three particular antigens (A29L, A35R, B6R), while Panel B presents the amino acid sequence of the target antigens along with a multiple sequence alignment with known MPXV and representative MPXV strain (MPXV Zaire-96-I-16), FDA-approved smallpox vaccines (Vaccinia virus Ankara, Acambis), and the specific smallpox vaccine used in China (Tiantan).
References
- 1.Moschese D., Pozza G., Giacomelli A., Mileto D., Cossu M., Beltrami M., et al. Natural history of human Monkeypox in individuals attending a sexual health clinic in Milan, Italy. J Infect. 2023;86(1):e18–e20. doi: 10.1016/j.jinf.2022.08.019. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Raccagni A., Candela C., Mileto D., Bruzzesi E., Canetti D., Bertoni C., et al. Breakthrough monkeypox infection among individuals previously immunized with smallpox or monkeypox vaccination. J Infect. 2023;86(2):154–225. doi: 10.1016/j.jinf.2022.12.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Edghill-Smith Y., Golding H., Manischewitz J., King L., Scott D., Bray M., et al. Smallpox vaccine-induced antibodies are necessary and sufficient for protection against monkeypox virus. Nat Med. 2005;11(7):740–747. doi: 10.1038/nm1261. [DOI] [PubMed] [Google Scholar]
- 4.Gong Q., Wang C., Chuai X., Chiu S. Monkeypox virus: a re-emergent threat to humans. Virol Sin. 2022;37(4):477–482. doi: 10.1016/j.virs.2022.07.006. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Fenner F., Breman J.G., Organization W.H. World Health Organization; 1978. Report on a visit to the People's Republic of China to consider matters relating to the certification of smallpox eradication, 14–30 July 1979.〈https://apps.who.int/iris/handle/10665/68275〉 [Google Scholar]
- 6.Hui X., Yutu J. The eradication of smallpox in Shanghai, China, October 1950–July 1951. Bull World Health Organ. 1981;59(6):913–917. [PMC free article] [PubMed] [Google Scholar]
- 7.Perez Duque M., Ribeiro S., Martins J., Casaca P., Leite P., Tavares M., et al. Ongoing monkeypox virus outbreak, Portugal, 29 April–23 May 2022. Euro Surveill. 2022;27(22) doi: 10.2807/1560-7917. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Tarín-Vicente E., Alemany A., Agud-Dios M., Ubals M., Suñer C., Antón A., et al. Clinical presentation and virological assessment of confirmed human monkeypox virus cases in Spain: a prospective observational cohort study. Lancet. 2022;400(10353):661–669. doi: 10.1016/S0140-6736(22)01436-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Banerjee-Guha S. Status of rural migrant workers in Chinese cities. Econ Political Wkly. 2011;46(26/27):33–37. 〈http://www.jstor.org/stable/23018638〉 [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
The information on the specific MPXV-specific antigens. Panel A shows the SDS-PAGE of three particular antigens (A29L, A35R, B6R), while Panel B presents the amino acid sequence of the target antigens along with a multiple sequence alignment with known MPXV and representative MPXV strain (MPXV Zaire-96-I-16), FDA-approved smallpox vaccines (Vaccinia virus Ankara, Acambis), and the specific smallpox vaccine used in China (Tiantan).