A new study has found preliminary evidence suggesting that the Omicron variant of SARS-CoV-2 can infect people who have been previously infected by other variants. Results of the study were posted on Medrxiv.org on Thursday.
The study found that although the emergence of previous variants – Beta and Delta were associated with a large increase in primary infections, they were not associated with an increase in the risk of reinfections. In other words, immunity gained from prior infections effectively prevented reinfections during waves caused by Beta and Delta variants of SARS-CoV-2.
“Although increases in the hazard of primary infection were observed following the introduction of both the Beta and Delta variants, no corresponding increase was observed in the reinfection hazard.”
In contrast, the new Omicron variant was associated with a lower rate of primary infections, but a higher rate of reinfections. According to lead author Juliet Pulliam “Recent reinfections have occurred in individuals whose primary infections occurred across all three waves, with the most having their primary infection in the Delta wave.” This suggests that Omicron evades immunity gained from prior infections better than Beta and Delta.
The lower incidence of primary infections by Omicron is a good sign, although it may not be cause for celebration. In a tweet, Dr. Pulliam said that the lower rate of primary infections “could be partially explained by vaccine rollout”.
In the last two weeks, Omicron has caused a sharp, almost exponential increase in the number of COVID-19 cases. According to South Africa’s National Institute for Communicable Diseases (NICD), “74% of all the virus genomes it had sequenced last month had been of the new variant”. Omicron is outcompeting other variants, including Delta and rapidly becoming the dominant SARS-CoV-2 variant.
Researchers now think that Omicron’s unique ability to evade acquired immunity may be the reason for it becoming the predominant strain. They however do not yet know if Omicron evades vaccine-derived immunity. According to WHO, more than 42% of the population globally are fully vaccinated. Omicron’s impact on global health and economy depends crucially on how effective vaccine-derived immunity will prove to be against it.
The study arrived at the worrying conclusion after measuring the rate of primary infections and reinfections associated with various SARS-CoV-2 variants until November 27 this year. To do this researchers used hospitalization data of 2,796,982 individuals collected through South Africa’s National Medical Surveillance System.
Read more about the study here. Please note that the paper is a preprint and has not been peer-reviewed as of 03-12-2021.