Studies support triple shots

Three new studies offer more evidence that Covid-19 vaccines are standing up to the Omicron variant, at least among people who received booster shots.

They are the first large United States studies to look at vaccine protection against Omicron. They echo previous research – including studies in Germany, South Africa and Britain – indicating that available vaccines are less effective against Omicron than earlier versions of the coronavirus, but also that booster doses rev up virus-fighting antibodies to increase the chance of avoiding symptomatic infection.

The first study looked at hospitalisations and emergency room and urgent care centre visits in 10 US states, from August to this month. It found that vaccine effectiveness was best after three doses of the Pfizer or Moderna vaccines in preventing Covid-associated emergency department and urgent care visits.

Protection from just two doses was lower, especially if six months had passed since the second dose.

A third dose was at least 90 per cent effective at preventing hospitalisations for Covid-19, both during the Delta and

Omicron outbreaks, the study also found.

The second study focused on Covid-19 case and death rates in 25 states from the beginning of April through to Christmas. People who were boosted had the highest protection against coronavirus infection, both during the time Delta was dominant and also when Omicron was taking over.

The two articles were published online by the US Centres for Disease Control and Prevention (CDC).

The Journal of the American Medical Association published the third study, also led by CDC researchers. It looked at people who tested positive for Covid-19 from December 10 to January 1 at more than 4600 testing sites across the US.

Three shots of the Pfizer and Moderna vaccines were about 67 per cent effective against Omicron-related symptomatic disease compared with unvaccinated people.

Two doses, however, offered no significant protection against Omicron when measured several months after completion of the original series, 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.