Risk and severity of SARS-CoV-2 reinfections during 2020-2022 in Vojvodina, Serbia: A population-level observational study

Abstract

Background: Data on the rate and severity of SARS-CoV-2 reinfections in real-world settings are scarce and the effects of vaccine boosters on reinfection risk are unknown.

Methods: In a population-level observational study, registered SARS-CoV-2 laboratory-confirmed Vojvodina residents, between March 6, 2020 and October 31, 2021, were followed for reinfection ≥90 days after primary infection. Data were censored at the end of follow-up (January 31, 2022) or death. The reinfection risk was visualized with Kaplan-Meier plots. To examine the protective effect of vaccination, the subset of individuals with primary infection in 2020 (March 6-December 31) were matched (1:2) with controls without reinfection.

Findings: Until January 31, 2022, 13,792 reinfections were recorded among 251,104 COVID-19 primary infections (5.49%). Most reinfections (86.77%, 11,967/13,792) were recorded in January 2022. Reinfections were mostly mild (99.17%, 13,678/13,792). Hospitalizations were uncommon [1.08% (149/13,792) vs. 3.66% (505/13,792) in primary infection] and COVID-19 deaths were very rare (20/13,792, case fatality rate 0.15%). The overall incidence rate of reinfections was 5.99 (95% CI 5.89-6.09) per 1000 person-months. The reinfection risk was estimated as 0.76% at six months, 1.36% at nine months, 4.96% at 12 months, 16.68% at 15 months, and 18.86% at 18 months. Unvaccinated (OR=1.23; 95%CI=1.14-1.33), incompletely (OR=1.33; 95%CI=1.08-1.64) or completely vaccinated (OR=1.50; 95%CI=1.37-1.63), were modestly more likely to be reinfected compared with recipients of a third (booster) vaccine dose.

Interpretation: SARS-CoV-2 reinfections were uncommon until the end of 2021 but became common with the advent of Omicron. Very few reinfections were severe. Boosters may modestly reduce reinfection risk.

Funding: No specific funding was obtained for this study.

Keywords: Boosters; COVID-19 vaccines; Reinfection risk; Reinfection severity; SARS-CoV-2.

Figure 1

Kaplan-Meier curve showing the cumulative probability of reinfection in the overall cohort (A), according to severity of primary infection (B), pandemic waves (C), gender (D), and age category (E) in Vojvodina, Serbia, March 2020–January 2022. The duration of pandemic waves was as follows: First pandemic wave: March 6–June 1, 2020; Second pandemic wave: June 2–October 6, 2020; Third pandemic wave: October 7, 2020–January 31, 2021; Fourth pandemic wave: February 1–July 23, 2021; Fifth pandemic wave: July 24–December 31, 2021.

Figure 2

Severity of COVID-19 in patients with reinfection (first and second episode) vs. patients without reinfection, in Vojvodina, Serbia, March 6, 2020–January 31, 2022.