Testing positive for omicron BA.1 plus two to three mRNA vaccines is the strongest protection against omicron BA.2 compared to being vaccinated with no past COVID-19 infections, a study published Sept. 21 in The Lancet Infectious Diseases found.
The researchers studied 37,732 positive COVID-19 cases among healthcare workers between March 27 and June 4, when omicron subvariant BA.2 was dominant in Canada and most positive tests were assumed to be BA.2 infections.
Of the workers who were vaccinated with either Pfizer's or Moderna's mRNA-based doses and likely had BA.1, their risk of another infection fell 96 percent, according to the study. Unvaccinated individuals who had a presumed BA.1 infection have a 72 percent risk reduction, and vaccinated people who have not had COVID-19 have a 46 percent risk reduction.
The lowest protective factor for future infection was having a previous, pre-omicron COVID-19 case, which garnered a 38 percent risk reduction.
Because there wasn't a measurable difference in risk reduction between a second and a third COVID-19 vaccine, the researchers concluded additional doses should be prioritized for "people who are more vulnerable globally."