It found vaccine effectiveness was best after three doses of the Pfizer or Moderna vaccines in preventing Covid-19-associated emergency department and urgent care visits. Protection dropped from 94 per cent during the Delta wave to 82 per cent during the Omicron wave. Protection from just two doses was lower, especially if six months had passed since the second dose.
The second study focused on Covid-19 case and death rates in 25 states from the beginning of April through 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 Centres for Disease Control and Prevention.
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, 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. - AP
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