A preliminary study from South Africa has suggested that the new Omicron SARS-CoV-2 variant causes a three-times increase in the chances of causing reinfection compared to the other variants of concern (VOC) such as the Beta and Delta variants. The study has been published as a preprint on medRxiv.¹

Researchers carried out a retrospective analysis of routine epidemiological surveillance data collected through South Africas National Notifiable Medical Conditions Surveillance System between 4th March 2020 and 27th November 2021. Around 2.8 million positive coronavirus samples in the country were analysed; these samples had tested positive at least 90 days before 27th November 2021. Suspected reinfections cases had two consecutive positive tests at least 90 days apart. The objective of the study was to find out the impact of the emergence of the Beta, Delta, and Omicron variants on the risk of reinfection in South Africa.

Out of the 2,796,982 positive Covid-19 cases, 35,670 were suspected to have reinfection. Increase in the risk of primary infection was observed in the Beta and Delta waves, but no corresponding increase was observed in the reinfection hazard.

Compared to the first wave, the estimated hazard ratio for reinfection vis a vis primary infection was lower during the Beta and Delta waves with a relative hazard ratio for wave 2 vs wave 1 as 0.75; the relative hazard ratio was 0.71 for wave 3 versus wave 1.

On the other hand, a reduction in the risk of primary infection and an increase in risk of reinfection has been seen with the new Omicron variant. Compared to the first wave, the estimated hazard ratio for reinfection vs primary infection from 1^st of  November 2021 to 27^th of November 2021 was 2.39. According to the National Institute of Communicable Diseases (NICD), South Africa, “An increase of reinfections rather than new infections would be an indication the new variant has developed the ability to evade natural immunity from previous infection.”

In contrast, no evidence of increased reinfection risk associated with circulation of Beta or Delta variants compared to the ancestral strain could be found “suggesting that the selective advantage that allowed these variants to spread derived primarily from increased transmissibility, rather than immune escape.”

Omicron has replaced the delta variant as the dominant strain in South Africa in a very short period of time, just around two weeks. This analysis of surveillance data from South Africa has provided preliminary evidence to suggest that the newly identified VOC Omicron has significant ability to evade natural immunity acquired from an earlier infection and thereby increase risk of reinfection. This increase is still ongoing and has a temporal association with the discovery of the new variant given the recent spike in the number of new reinfection cases per day. Whether it also escapes vaccine-induced immunity is a question that will be answered only with time.

Reference

1. Pulliam JRC, et al. Increased risk of SARS-CoV-2 reinfection associated with emergence of the Omicron variant in South Africa. medRxiv, December 02, 2021, doi: https://doi.org/10.1101/2021.11.11.21266068.