. 2021 May 27;32(1):e2260. doi: 10.1002/rmv.2260

TABLE 2.

Summary of included studies and primary outcome results

First author; country; population	Participants ^a Follow‐up	Author reported primary outcomes
Abu‐Raddad 2021 ⁷ (preprint); Qatar; General population	N = 43,044 antibody‐positive at baseline	Risk of reinfection (confirmed by WGS) ^b : 0.10% (95% CI: 0.08%–0.11%) Risk over time (any reinfection): Incidence rate of reinfection by month of follow‐up did not show any evidence of waning of immunity over seven months of follow‐up
	Median f/u: 114 days (3.8 months)
	Maximum f/u: 242 days (8.1 months)
Hall 2021 ⁸ (preprint); United Kingdom; HCWs	N = 6614 antibody‐positive at baseline	Adjusted odds ratio of reinfection comparing antibody or PCR‐positive group with negative group • ‘Probable’ reinfection ^c : aOR: 0.01 (95% CI 0.00–0.03) • All ‘possible’ and ‘probable’ reinfections: aOR: 0.17 (95% CI: 0.13–0.24) • Symptomatic reinfection: aOR: 0.08 (95% CI 0.05–0.13)
	Median f/u: 202 days (6.7 months)
	Maximum f/u: 227 days (7.6 months)
Hanrath 2020 ⁹ United Kingdom; HCWs	N = 1038 PCR and/or antibody‐positive at baseline	Symptomatic reinfection: A positive PCR test was returned in 0/1038 (0% [95% CI: 0–0.4) of those with previous infection, compared with 290/10,137 (2.9% [95% CI: 2.6–3.2) of those without (p < 0.0001 χ2 test)
	Median f/u: 173 days (5.8 months)
	Maximum f/u: 229 days (7.6 months)
Hansen 2021 ¹⁷ Denmark; General population	N = 11,068 PCR positive at baseline	Main analysis: aRR (any reinfection): 0.20 (0.16–0.25). This represents 72 reinfections out of 1,346,920 person‐days in PCR‐positive group, compared with 16,819 new infections out of 62,151,056 person‐days in PCR‐negative group Additional cohort analysis (that includes all infection periods): aRR = 0.21 (0.18–0.25) by age group: 0–34 years: aRR = 0.17 (0.13–0.23) 35–49 years: aRR = 0.20 (0.14–0.28) 50–64 years: aRR = 0.19 (0.13–0.27) ≥65 years: aRR = 0.53 (0.37–0.75)
	Median f/u: 122 days (4.1 months)
	Maximum f/u: 295 days (9.8 months)
Harvey 2020 ¹⁰ (preprint); United States; General population	N = 378,606 PCR positive at baseline	Ratio of positive NAAT results (comparing patients who had a positive antibody test at index vs. those without) ^d : 2.85 (95% CI: 2.73–2.97) at 0‐30 days; 0.67 (95% CI: 0.6–0.74) at 31–60 days; 0.29 (95% CI: 0.24–0.35) at 60–90 days; 0.10 (95% CI: 0.05–0.19) at >90 days; note that NAAT positivity at <90 days is likely due to prolonged viral shedding
	Median f/u: 54 days (1.8 months)
	Maximum f/u: 92 days (3.1 months)
Jeffery‐Smith 2021 ¹³ United Kingdom; Staff &residents at care homes	N = 88 PCR and/or antibody‐positive at baseline	Relative risk (any reinfection): 0.04 (95% CI: 0.005–0.27) This represents 1 reinfection out of 88 in seropositive group compared with 22/73 in seronegative group
	Mean f/u: 120 days (4 months)
	Maximum f/u: Unclear
Krutikov 2021 ¹⁴ (preprint); United Kingdom; Staff & residents at care homes	N = 634 antibody‐positive at baseline	Relative adjusted hazard ratios (any reinfection):Residents of care home: aHR = 0.15 (0.05–0.44) ^e Staff of care home: aHR = 0.39 (0.19–0.82) ^e
	Median f/u: 79 days (2.6 months)
	Maximum f/u: 300 days (10 months)
Lumley 2021 ¹¹ United Kingdom; HCWs	N = 1265 antibody‐positive at baseline	IRR ^f (any reinfection): 0.12 (95% CI: 0.03–0.47; p = 0.002); 2/1265 seropositive (both asymptomatic reinfections) and N = 223/11,364 seronegative had positive PCR. Symptomatic reinfection: Incidence was 0.60 per 10,000 days at risk in seronegative HCWs; there were no symptomatic infections in seropositive HCWs Adjusted IRR ^g : 0.11 (95% CI: 0.03–0.44; p = 0.002) (any reinfection)
	Median f/u: 139 days (4.6 months)
	Maximum f/u: 217 days (7.2 months)
Perez 2021 ¹² (preprint); Israel; General population	N = 149,735 PCR positive at baseline	Overall reinfection risk: 0.1% (any reinfection between Mar 2020 and Jan 2021) This represents 154 individuals who had two positive tests at least 100 days apart out of 149,735 individuals with a record of a prior positive PCR test
	Median f/u: 165 days (5.5 months)
	Maximum f/u: Approx. 325 days ^h (10.8 months)
Pilz 2021 ¹⁶ Austria; General population	N = 14,840 PCR positive at baseline	Odds ratio: 0.09 (95% CI: 0.07–0.13) (any reinfection) This represents 40 reinfections out of 14,840 individuals PCR positive in the first wave (0.27%) compared with 253,581 infections out of 8,885,640 (2.85%) in the remaining general population
	Median f/u: 210 days (7 months)
	Maximum f/u: 300 days (10 months)
Sheehan 2021 ¹⁵ (preprint); United States; General population	N = 8845 PCR positive at baseline	Protective effectiveness (any reinfection): 78.5% (95% CI: 72.0%–83.5%) ⁱ Protective effectiveness against symptomatic infection: 83.1% (95% CI: 75.1%–88.5%)
	Median f/u: 131 days (4.4 months)
	Maximum f/u: 269 days (9 months)

Note: ‘Any’ reinfection—all reinfections, both symptomatic and asymptomatic. Numbers rounded to two decimal points. No cases were identified on the basis of antigen testing. The longest duration of follow‐up was not stated in all studies or was provided only as an approximate estimate; when not stated, duration of follow‐up was inferred from figures or tables within the study.

Abbreviations: aHR, adjusted hazard ratio; aOR, adjusted odds ratio (adjusted for week group); ARR, adjusted rate ratio; CI, confidence interval; f/u, follow‐up; HCW, healthcare worker; IRR, incidence rate ratio; NAAT, nucleic acid amplification test; WGS, whole genome sequencing.

^{^a}

In the baseline antibody and or PCR‐positive group (‘seropositive’ or prior positive cohort).

^{^b}

Based on cases with WGS confirming the first and second infections were from different viral strains (N = 16).

^{^c}

‘Possible’ reinfection was defined as a participant with two PCR‐positive samples ≥90 days apart with available genomic data, or an antibody‐positive participant with a new positive PCR at least 4 weeks after the first antibody‐positive result. A ‘probable’ case additionally required supportive quantitative serological data and or supportive viral genomic data from confirmatory samples.

^{^d}

NAAT used as proxy; includes all symptomatic reinfections and prolonged viral shedding, comparing patients who had a positive antibody test at index versus those with a negative antibody.

^{^e}

Multivariate analysis of risk of PCR‐positive infection by baseline antibody status, stratified by LTCF and adjusted for sex and age.

^{^f}

IRR is the relative incidence of subsequent positive SARS‐CoV‐2 PCR tests and symptomatic infections comparing antibody‐positive and antibody‐negative groups at baseline.

^gAfter adjustment for age, gender and month of testing or calendar time as a continuous variable.

^{^h}

The midpoint of a range of follow‐up dates was taken (300–349 days).

^ⁱ

Authors report effectiveness with the following calculation: 1−([56/8845]/[4163/141480]).