Vaccines protect against infection from the Omicron subvariant but only for a short time

For example, two doses of the COVID vaccine reduce the risk of disease and mild illness from the rising BA.2 subvariant, but protection is short-lived.

In many countries, the Omicron subvariant BA.2 is replacing its sister version, BA.1, as the dominant form of SARS-CoV-2, prompting scientists to wonder if the COVID-19 pandemic is about to devastate these regions once more. However, according to a study1 published on March 13th, mRNA vaccines provide comparable levels of protection against the two strains — though protection against SARS-CoV-2 infection and symptomatic disease fade within months of a third dose.

For months, researchers have known that the BA.1 subvariant avoids much of the protection provided by mRNA vaccines against the mild-to-moderate disease. Scientists quickly realized that BA.2 spreads faster than BA.1, but it wasn't clear whether the newcomer would also be better at evading vaccines.

"The fear was that BA.2 would be even worse than BA.1," says Laith Abu-Raddad, an infectious-diseases epidemiologist at Weill Cornell Medicine–Qatar in Doha and a co-author of the study.

Abu-Raddad and his colleagues conducted a large observational study on vaccination records and SARS-CoV-2 test results obtained from Qatar's healthcare system. They discovered that Qataris who received two doses of either the Pfizer–BioNTech or Moderna mRNA-based vaccines enjoyed several months of significant protection against symptomatic disease caused by either BA.1 or BA.2. However, after only 4–6 months, protection dwindled to around 10%, implying that the vaccines prevented only 10% of the cases that would have occurred if all of the individuals had been unvaccinated.

Protection against BA.2 did not appear to wane as quickly as protection against BA.1, and a booster shot restored protection against symptomatic infection by either subvariant to 30–60 percent. Surveillance data from the United Kingdom show a similar pattern: vaccine effectiveness against symptomatic COVID-19 is less than 20% for both subvariants 25 weeks or more after a second dose. Still, it rises to roughly 70% 2–4 weeks after a third dose.

The researchers also examined the level of protection provided by mRNA vaccines against severe disease. Still, to do so, they had to pool data on BA.1 and BA.2 cases — a necessary measure because Qatar's population is heavily skewed toward young people, making severe COVID-19 cases uncommon. Only after pooling cases did the researchers have enough to produce meaningful results.

This study found that even in people who had only received two vaccine doses, protection against severe disease remained at 68 percent or higher for at least seven months and increased to more than 80 percent after a booster dose. Because 70-80 percent of the pooled cases were BA.2, Abu-Raddad believes vaccines still provide a high level of protection against severe disease in the face of rising BA.2 levels.