Study Study Study Outcome measures of protection or risk Outcome measures of protection or risk Outcome measures of protection or risk
Total size (enrolled; before exclusions) Time period Reported lineage Reported outcome measure (protection, risk, reinfection rate) Repeat infection outcome (selected comparisons, terminology as reported) Severe COVID-19 outcome (selected comparisons, terminology as reported)
Primary publications Primary publications Primary publications Primary publications Primary publications Primary publications Primary publications
Hansen et al. 2021 Non-vaccinated individuals Denmark (141)
~ 4 million individuals
Feb 26, 2020–Dec 31, 2020
None
Protection
Protection against repeat infection in those11Derived as 1− adjusted relative risk. The rates of infection during the second surge were compared across those with a positive or negative PCR test from the first surge. The calculated the rate of infection was calculated as the number of individuals with positive PCR tests during the second surge divided by the cumulative number of person-days at risk < 65 years: 80.5% (95% CI 75.4–84.5) ≥ 65 years: 47.1% (96% CI 24.7–62.8) Not assessed
Abu-Raddad et al. 2021 Non-vaccinated individuals22Qatar launched its vaccination campaign on December 21, 2020, around the time this study was concluded (December 31, 2020), so very few individuals had been vaccinated at time of this study. Qatar (142)
192,984 individuals
April 16, 2020–Dec 31, 2020
None
Protection
Efficacy of natural infection against reinfection33Derived as 1− the ratio of the incidence rate of reinfection in the antibody-positive cohort to the incidence rate of infection in the antibody-negative cohort. 95.2% (95% CI: 94.1–96.0) Not assessed Of 129 cases with good or some evidence of reinfection, one reinfection was severe, two were moderate, and none were critical or fatal
Hall et al. 2021 Non-vaccinated and vaccinated individuals UK (143)
30,625 individuals
June 18, 2020–Jan 11, 2021
Not specified B.1.1.7
Risk
Risk of reinfection causing44Derived as 1− adjusted incident rate ratio. COVID-19 symptoms: aIRR 0.074 (95% CI 0.06–0.10) All events (COVID-19 symptoms, other symptoms, asymptomatic): aIRR 0.159 (95% CI 0.13–0.19) Not assessed
Lumley et al. 2022 Non-vaccinated and vaccinated individuals UK (144)
13,109 individuals
March 27, 2020–Feb 28, 2021
Non-S-gene target failure B.1.1.7
Risk
Risk of PCR-positive result (symptomatic or asymptomatic) in Unvaccinated seropositive55Compared incidence in each follow-up group to unvaccinated seronegative healthcare workers.: aIRR 0.02 (95% CI 0.01–0.18) Not assessed
Abu-Raddad et al. 2021 Non-vaccinated and vaccinated individuals Qatar (145)
193,233 individuals
Before Nov 1, 2020–March 3, 2021
B.1.1.7 Variants of unknown status
Protection
Efficacy of natural infection against reinfection with66Derived as 1− the ratio of the incidence rate of reinfection in the PCR-confirmed (or antibody-positive) cohort to the incidence rate of infection in the antibody-negative cohort. B.1.1.7, prior PCR-confirmed infection: 97.5% (95% CI 95.7–98.6) B.1.1.7, prior antibody-positive result: 97.0% (95% CI 92.5–98.7) Unknown variant, prior PCR-confirmed infection: 92.2% (95% CI: 90.6–93.5) Unknown variant, prior antibody-positive result: 94.2% (95% CI: 91.8–96.0) Not assessed
Chemaitelly, H et al. 2021 Unvaccinated individuals (146) Qatar
380,914 individuals
Before Jan 1, 2021–April 21, 202177This timeframe coincided with the beginning of the decline of the B.1.1.7 wave and the rapid expansion of the B.1.351 wave that peaked early April 2021.
B.1.351 B.1.1.7 Variants of unknown status
Protection
Efficacy of natural infection against reinfection with88Derived as 1− the ratio of the incidence rate of reinfection in the cohort of individuals with a prior PCR-confirmed infection to the incidence rate of infection in the antibody-negative cohort. B.1.351: 92.3% (95% CI: 90.3–93.8) B.1.1.7: 97.6% (95% CI 95.7–98.7) Variants of unknown status: 87.9% (95% CI: 84.7–90.5) Not assessed
Nordström et al. 2022 Non-vaccinated and vaccinated individuals Sweden (147)
~3.5 million individuals (3 cohorts)
March 20, 2020–Sept 5, 2021
Alpha B.1.1.7 Beta B.1.351 Gamma P.1 Delta B.1.617.2
Risk
Risk of reinfection in those with Natural immunity99Calculated vs no immunity and after 3 months of follow-up.:
aHR 0.05 (95% CI 0.05–0.05) One-dose hybrid immunity1010Calculated vs natural immunity and during the first 2 months of follow-up.:
aHR 0.42 (95% CI 0.38–0.47) One-dose hybrid immunity1111Calculated vs natural immunity and after 2 months of follow-up.:
aHR 0.55 (95% CI 0.39–0.76) Two-dose hybrid immunity, overall1212Calculated vs natural immunity.:
aHR 0.34 (95% CI 0.31–0.39)
Risk of hospitalization (HR) Two-dose hybrid immunity1313Calculated vs natural immunity.: 0.10 (95% CI 0.04–0.22)
Altarawneh et al. 2022 Non-vaccinated and vaccinated individuals Qatar (148)
~2.3 million individuals
March 23, 2021–Nov 18, 2021
Alpha Beta Delta Omicron
Protection
Effectiveness of previous infection in preventing reinfection with1414Derived as 1− odds ratio of prior infection in cases (PCR-positive persons with variant infection) versus controls (PCR-negative persons)) Alpha: 90.2% (95% CI 60.2–97.6) Beta: 85.7% (95% CI 75.8–to 91.7) Delta: 92.0% (95% CI 87.9–94.7) Omicron: 56.0% (95% CI 50.6– 60.9) Effectiveness of previous infection in preventing severe, critical or fatal disease caused by Alpha: 69.4% (95% CI −143.6–96.2) Beta: 88.0% (95% CI 50.7–97.1) Delta: 100% (95% CI 43.3–100) Omicron: 87.8% (95% CI 47.5–97.1)
Pulliam et al. 2022 Non-vaccinated and vaccinated individuals South Africa (149)
~2.9 million individuals
March 4, 2020–Jan 31, 2022
Beta (B.1.351) Delta (B.1.617.2) Omicron (B.1.1.529)1515Period of Omicron emergence: November 1, 2021 to November 30, 2021.
Risk
Risk of reinfection during1616Estimated relative hazard ratios for reinfection during specified wave versus primary infection during the first wave. Wave 2 (Beta-driven) versus Wave 1: relative HR 0.71 (95% CI 0.60–0.85) Wave 3 (Delta-driven) versus Wave 1: relative HR 0.54 (95% CI 0.45–0.64) Wave 4 (Omicron-driven) versus Wave 1: relative 1.70 (95% CI 1.44–2.04) Not assessed
Guedes et al. 2023 Non-vaccinated and vaccinated individuals Brazil (150)
25,750 real-time RT-PCR tests performed
March 10, 2020–March 20, 2022
Pre-VOC Gamma Delta Omicron
Reinfection rate
Reinfection rate during the Omicron variant period: 1717Calculated as number of reinfection cases before and after the Omicron variant considering the total accumulated number of SARS-CoV-2 infections in both periods. Before 0.8% vs after 4.3%;
p<0.001
Not assessed 281/281 reinfections were mild
Chemaitelly et al. 2022 (151) Unvaccinated individuals Qatar
Up to 3.3 million individuals
Feb 28, 2020– June 5, 20221818Three individual studies (pre-Omicron reinfection, Omicron reinfection, COVID-19 severity reinfection) spanning different time periods.
Pre-Omicron (ancestral, Alpha, Beta, Delta) Omicron (BA.1, BA.2, BA.4, BA.5)
Protection
Effectiveness of pre-Omicron primary infection1919Derived as 1−adjusted hazard ratio, where the hazard ratio compared incidence of infection in both cohorts. Incidence rate of infection in each cohort defined as the number of identified infections divided by the number of person-weeks contributed by all individuals in the cohort. Against pre-Omicron reinfection: 85.5% (95% CI: 84.8–86.2%) Effectiveness peaked at 90.5% (95% CI 88.4–92.3%) in the 7th month after the primary infection, waning to ~70% by the 16th month Against Omicron reinfection: 38.1% (95% CI 36.3–39.8%), declining with time since primary infection Effectiveness of pre-Omicron primary infection2020Cox regression analysis. Severity, criticality, and fatality defined as per WHO guidelines. Against severe, critical or fatal COVID-19 due to Omicron reinfection:
88.6% (95% CI 70.9–95.5) Against severe, critical, or fatal COVID-19 reinfection (irrespective of the variant of primary infection or reinfection):
97.3% (95% CI 94.9–98.6)
Bowe et al. 2022 Non-vaccinated and vaccinated individuals USA (152)
~ 5.8 million individuals
March 1, 2020–June 25, 2022
Pre-Delta Delta Omicron
Risk
Not assessed Risk of all-cause mortality (HR)2121Calculated for reinfection vs no reinfection. 2.17 (95% CI 1.93–2.45) Risk of hospitalization (HR) 3.32 (95% CI 3.13–3.51)
Yang et al. 2023 Non-vaccinated and vaccinated individuals Malaysia (153)
482 individuals
Jan 31, 2022–Jul 31, 20222222The Omicron-dominant period in Malaysia was estimated to start from early February 2022.
Non-Omicron Omicron
Risk
Risk of reinfection in those with Pre-Omicron natural infection2323Calculated vs Omicron-dominant period.: aHR 0.41 (95% CI 0.27–0.62) Not assessed
Meta-analyses
Stein et al 2023. Global systematic review and meta-analysis of 65 studies from 19 countries (154)
Various
Up to Sept 31, 2022
Ancestral Mixed Alpha (B.1.1.7) Beta (B.1.351) Delta (B.1.617.2) Omicron BA.1 variants
Protection
Pooled estimate of protection from past infection (with various variants) against reinfection with Ancestral: 84.9 (95% UI 72.8–91.8) Alpha: 90.0% (95% UI 54.8–98.4) Beta: 85.7% (95% UI 83.4–87.7) Delta: 82.0 (95% UI 63.5–91.9) Omicron BA.1: 45.3% (95% UI 17.3–76.1) Pooled estimate of protection against severe disease caused by Ancestral: 78.1% (95% UI 34.4–96.5) Alpha: 79.6% (95% UI 43.3–95.3) Beta: 88% (95% UI 50.7–97.1)2424Single study. Delta: 97.2% (95% UI 85.2–99.6) Omicron BA.1: 81.9% (95% UI 73.8–88.0)