Currently available COVID-19 vaccines are much less effective against the Delta variant of the virus, though they still prevent serious illness in the vast majority of those inoculated, a study published Thursday by JAMA Network Open found.
In the analysis of vaccinated persons in
However, by mid-August, vaccine effectiveness declined to 78% before rebounding slightly to 81% by October, the researchers said.
By August, the Delta variant was involved in nearly 95% of infections in
"Although the vaccines are imperfect, they are still very effective at preventing severe disease and hospitalization," study co-author
"Masking, social distancing and frequent testing remain other tools to help slow the spread," said Keegan, a research assistant professor of epidemiology at the
New variants of viruses such as COVID-19 and the seasonal flu emerge as they circulate in the population, according to the
These variants have genetic mutations that change how they are structured, and the Delta strain has seven such changes to its spike protein, which is targeted by the vaccines, the agency said.
In comparison, the new Omicron variant has 32 mutations to the spike protein.
The mutations are sufficient to make it less susceptible to the vaccines, research published earlier this month found.
The variant, which first emerged in
It, along with the fast-spreading Omicron variant, are fueling an ongoing winter surge in cases across the northern half of the country, according to the
For this study, the
Of the more than 3.2 million adults living in the state, 1.7 million, or 67%, were fully vaccinated against the virus during the study period, meaning they had either received the one-dose
The vaccines were most effective at preventing serious illness from COVID-19 in mid-May, when the Delta variant accounted for fewer than 10% of cases across the state, the data showed.
As the variant became more common in
The findings "should serve as a caution throughout
"This reduction in the effectiveness of available vaccines associated with the arrival of novel variants of concern, rather than waning immunity, is concerning," they said.
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