Booster shots of coronavirus disease (Covid-19) vaccines are instrumental in resisting the highly transmissible Omicron variant of the virus, showed three studies released by the United States Centers for Disease Control and Prevention (CDC) on Friday. Notably, these are the first large-scale studies in the US that looked at vaccine protection against the Omicron variant of the coronavirus.

Echoing earlier studies, new research indicates pre-existing Covid-19 vaccines are less effective against Omicron than earlier variants of the SARS-CoV-2 coronavirus, but at the same time, booster shots significantly improve protection from Omicron.

There were three studies conducted in this regard. The articles for the first two were published online by the CDC while the third one, also led by CDC researchers, was published by the Journal of the American Medical Association.

The first study looked at Covid-associated hospitalisations and visits to emergency rooms and urgent care centers from August last year to this month in as many as 10 states in the US.

Also Read | Covid-19: Do multiple boosters ‘exhaust’ our immune response?

It found that vaccine efficacy was the best in preventing emergency department and urgent care visits after three doses of the Pfizer or Moderna vaccines. However, the high protection rate of 94 per cent during the Delta wave dropped to 82 per cent during the Omicron wave, proving that protection from two doses of the vaccine waned if six months had passed since the second dose.

The second study, on the other hand, looked at the Covid-19 case count and the death rates in 25 US states from the beginning of April through Christmas. This one showed that people who had been administered booster doses had the highest level of protection against a potential coronavirus infection, both during the time that Delta was dominant and also when Omicron was taking over.

The third study looked at people who tested positive for Covid-19 from December 10 to January 1 at more than 4,600 testing sites across the US.

It showed that three shots of the vaccines were about 67 per cent effective against Omicron-related symptomatic coronavirus disease compared to unvaccinated people. Two doses, however, offered no significant protection against Omicron, the researchers found.

“It really shows the importance of getting a booster dose,” said the CDC’s Emma Accorsi, one of the study’s authors.

The urgency for vaccinations and booster doses has been rising even amid reports that Omicron causes milder disease than earlier variants. Yet, many hospitals in the US remain overwhelmed by the sheer numbers of Omicron-infected patients, making prevention a key part of the battle against the coronavirus.

Also Read | WHO classifies 4 priority risk groups for Covid vaccine booster doses, says most vulnerable should be jabbed first

But booster doses remain as controversial as ever, since many low- and middle-income countries have been unable to secure even first shots for their populations.

Covax, the World Health Organisation (WHO)-backed program to distribute doses equitably around the globe, recently reached the milestone of delivering one billion doses, while more than 500 million have been administered in the US alone. The WHO, however, has not officially endorsed the use of boosters yet, except for vulnerable populations, such as the sick and elderly.

(With inputs from agencies)