In a recent study posted to the medRxiv* preprint server, researchers evaluated the incidences of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfections, hospitalizations, and death among the previously infected population of the Abruzzo region in Italy.
Study: Risk of SARS-CoV-2 reinfection 18 months after primary infection: population-level observational study. Image Credit: grebeshkovmaxim/Shutterstock
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
Reinfections due to SARS-CoV-2 are rare, especially after primary infection and among the vaccinated population. Yet, ascertaining how long natural immunity lasts, protects against SARS-CoV-2 new variant of concern (VOC) Omicron (BA.1/B.1.1.529), and the efficacy of vaccines in reducing reinfection rates is of utmost importance.
About the study
In the present retrospective study covering the entire population of the Abruzzo region in Italy, researchers estimated the incidence of SARS-CoV-2 reinfections in the cohort of 1,293,941 subjects from March 2020 to November 2021. After all the exclusions, they included 84,907 subjects with primary infection in the analysis, of which 2829 subjects had a follow-up time of longer than 18 months. The mean time after the primary infection was 334 days.
The data extraction was done on January 14, 2022, allowing more than 45 days of follow-up. They gathered the official vaccination data, plus coronavirus disease 2019 (COVID-19) demographic, hospital, and co-pay exemption datasets of the National Healthcare System; to merge with each participant's data using an encrypted fiscal code.
During the analyses, researchers considered vaccination status, predominant SARS-CoV-2 strain (pre-Omicron vs. Omicron from December 28, 2021, onwards), demographic and clinical characteristics, and time-lapsed after primary infection.
Subjects were classified as 'vaccinated' if they received one or more doses of the BNT162b2, ChAdOx1nCoV-19, mRNA-1273, or JNJ-78436735 vaccines, ≥14 days before the reinfection. Reinfection was defined by at least one SARS-CoV-2-negative reverse transcriptase-polymerase chain reaction (RT-PCR) test between two positive RT-PCR tests 45 days apart.
Study findings
The authors noted 152,986 infections among the 1,293,941 residents in the Abruzzo Region from the beginning of the COVID-19 pandemic. However, among 84,907 previously infected subjects, SARS-CoV-2 reinfections occurred in barely 260 individuals with an overall incidence rate of 0.31%. Furthermore, there were only two hospitalizations due to COVID-19, and one 73-old female participant died.
The reinfection rate was significantly higher among females and younger subjects; likewise, unvaccinated individuals were also prone to reinfections with reinfection rates of 0.50% vs. 0.25% among those who received two or more than two vaccine doses.
Regarding reinfection rates in 317 days of the pre-Omicron period, it was merely 0.4 reinfections per day. When the Omicron wave began December 2021 onwards, the rate of reinfections was markedly higher during the first 17 days, with 8.5 reinfections per day. Notably, the reinfection rate remained 0.32% after 18 months up to 22 months from the primary infection and did not even vary by baseline comorbidities.
When the analysis was stratified based on circulating variant, the effectiveness of two or more vaccine doses was substantially higher in the pre-Omicron period than during the Omicron dominance period.
Conclusions
To conclude, the study findings were consistent with previous reports suggesting that the risk of SARS-CoV-2 reinfection and severe or lethal COVID-19 was low among those who had recovered from primary infection, indicating that natural immunity conferred by primary infection lasts beyond 12 months.
Since the risk of death was almost zero, a marked increase in reinfections during the Omicron wave was not worrisome. Although COVID-19 vaccines drastically reduced the likelihood of reinfection in both pre-Omicron and Omicron times, a careful evaluation of the risk-benefit profile of multiple vaccine doses is warranted.
This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources
Article Revisions
- May 12 2023 - The preprint preliminary research paper that this article was based upon was accepted for publication in a peer-reviewed Scientific Journal. This article was edited accordingly to include a link to the final peer-reviewed paper, now shown in the sources section.
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