Icelandic study suggests SARS-CoV-2 Omicron reinfection more common than previously thought

By Pooja Toshniwal PahariaAug 9 2022Reviewed by Benedette Cuffari, M.Sc.

In a recent study published in JAMA Network Open, researchers estimate the proportion of Icelandic residents reinfected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant of concern (VOC).

Study: Rate of SARS-CoV-2 Reinfection During an Omicron Wave in Iceland. Image Credit: angellodeco / Shutterstock.com

Background

The coronavirus disease 2019 (COVID-19) was announced as a global pandemic on March 11, 2020. The SARS-CoV-2 Omicron VOC, which is currently the dominant circulating strain throughout the world, has higher immune-evasive characteristics as compared to previous VOCs, thus increasing the likelihood of SARS-CoV-2 reinfection.

The relative immune protection induced by previous SARS-CoV-2 infection against reinfection with the Omicron VOC has been reported to be 56% as compared to 92% for reinfections with the Delta VOC. However, the risk of reinfection with the Omicron VOC at a population level needs further investigation.

About the study

In the current population-based cohort study, researchers estimate the risk of SARS-CoV-2 reinfection with the Omicron VOC in Iceland.

Individuals with a prior history of SARS-CoV-2 infection were assessed for the development of reinfection during the period of Omicron dominance in Iceland. To this end, the current study was conducted between December 1, 2021, which was the date of the first reported Omicron case among Icelandic residents, and February 13, 2022, which is when the study ended.

The researchers obtained data on the status of COVID-19 vaccinations and previous SARS-CoV-2-positive reverse transcription-polymerase chain reaction (RT-PCR) analysis reports for their analysis.

An individual was considered to be reinfected with SARS-CoV-2 if he/she had tested positive for COVID-19 by RT-PCR more than 60 days from the initial SARS-CoV-2-positive RT-PCR result. Logistic regression models were used for estimating the proportion of SARS-CoV-2 reinfections based on the days elapsed from the previous SARS-CoV-2-positive RT-PCR reports, participants’ age, and the status of COVID-19 vaccinations, and the calculated odds ratios (OR).

Study findings

A total of 11,536 COVID-19 cases confirmed by RT-PCR were considered for the analysis. The average age of the study participants was 34 years, 51% of whom were men. About 26% of the study participants had received at least one dose of a COVID-19 vaccine.

The average number of days elapsed from the initial SARS-CoV-2-positive RT-PCR test result was 287 days. SARS-CoV-2 reinfections were reported in 12% of individuals during the period of Omicron predominance.

Among participants who had received at least one COVID-19 vaccine dose, 12% were reinfected with SARS-CoV-2. This was comparable to 11% who had received at least two COVID-19 vaccine doses. The rate of reinfection was highest at 15% among individuals between the ages of 18 and 29 years, with fewer reinfections observed among elder individuals.

The odds of SARS-CoV-2 reinfection were proportionately higher with days elapsed from the previous SARS-CoV-2 infection, with an OR of 1.6 for 18 months as compared to three months. Moreover, the risk of reinfection was greatest among individuals who had received at least two vaccine doses as compared to those who had received one vaccine dose with an OR of 1.4.

Conclusions

The study findings showed that a considerable proportion of individuals developed SARS-CoV-2 reinfections in the initial 74 days of Omicron predominance in Iceland, with the highest rate of reinfections observed among 18- to 29-year-old individuals. A directly proportional correlation was observed between the probability of reinfection and the number of days elapsed from the previous SARS-CoV-2 infections.

Unexpectedly, two or more vaccine doses were related to a marginally greater likelihood of reinfection as compared to one or fewer vaccine doses.

The study findings must be cautiously interpreted, as the study had several limitations. These included the lack of adjustments for the composite associations between previous SARS-CoV-2 infections, eligibility criteria for COVID-19 vaccination, and the presence of underlying medical conditions.

By December 1, 2021, all individuals above 12 years of age met the eligibility criteria for receiving two COVID-19 vaccine doses without charges. By this point, over 71% of the population of Iceland had been vaccinated against COVID-19, which is comparable to only 26% of the study cohort of individuals with prior COVID-19 history.

The study findings indicate that SARS-CoV-2 reinfection may be more common than originally expected. In the current scenario, it is important to determine whether Omicron infection would produce enhanced immune protection against reinfection with Omicron as compared to other SARS-CoV-2 VOCs.