The COVID-19 vaccines currently available in the United States were less effective at preventing against infection after the Omicron variant of the virus emerged in November, according to data released Friday by the Centers for Disease Control and Prevention.
However, they still lower the risk for serious illness among those who are fully vaccinated, as well as those who have received booster doses, according to the agency.
In October and November, when the Delta variant was still the predominant one in circulation nationally, the shots available from Johnson & Johnson, Moderna and Pfizer-BioNTech, collectively, offered 93% protection against infection, the data showed.
By December, though, when the Omicron strain accounted for more than 70% of new cases across the country, vaccine effectiveness dipped to 80%, the agency said.
Still, through the end of November, the unvaccinated made up 75% of all reported COVID-19 deaths across the 22 states and three metropolitan areas -- New York City, Seattle and Washington, D.C. -- included in the analysis, according to the CDC.
The vaccine effectiveness figures are based on infection rates among those fully vaccinated, meaning they had received both doses of the Moderna and Pfizer-BioNTech shots or the single-dose Johnson & Johnson product, the CDC said.
A booster dose of all three vaccines increased effectiveness at protecting against infection to 82%, a separate analysis, also released Friday, found.
"COVID-19 vaccines reduced risks for ... infection and COVID-19-associated death during periods of Delta variant predominance and infection risk during Omicron variant emergence, the CDC researchers wrote.
"Because of reporting lags, the influence of the Omicron variant on COVID-19-associated deaths by vaccination status in December could not be evaluated," they said.
The Omicron variant of COVID-19 is believed to be more contagious and resistant to currently available vaccines, experts said.
As a result of the reduced effectiveness of the vaccines at preventing infection with Omicron, the regions included in the analysis reported a 23% increase in new cases among those fully vaccinated after the strain emerged, according to the agency.
Case figures also increased six-fold among those fully vaccinated and boostered, the agency said.
However, after the emergence of Omicron, the unvaccinated were four times as likely to be hospitalized due to COVID-19 than those who were within six months of becoming fully vaccinated, the data showed.
They were also 25% more likely to require hospital treatment following infection than those who were more than six months out from the final vaccine dose and twice as likely to be hospitalized than those who had received a booster, according to the CDC.
Even with Omicron, the vaccines were 90% effective at preventing severe illness, as measured by hospitalizations, the agency said.
"These findings underscore the importance of receiving a third dose of [the Moderna or Pfizer-BioNTech vaccines] to prevent both moderately severe and severe COVID-19, especially while the Omicron variant is the predominant circulating variant," the CDC researchers wrote.
"All unvaccinated persons should get vaccinated as soon as possible," they added.
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