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A study published by the Centers for Disease Control and Prevention in the US has shown that the effectiveness of the Pfizer and Moderna coronavirus vaccines dropped from 91 percent before the Delta variant became dominant, to 66 percent afterwards.
The US health protection agency, CDC, has been examining the real-world performance of the two vaccines since they were first authorised among healthcare personnel, ambulance staff and other frontline workers.
Thousands of workers across six states were tested weekly, and immediately after the onset of Covid-19 symptoms, allowing researchers to estimate the impact of the vaccines against symptomatic and asymptomatic infection.
Vaccine effectiveness was estimated at 91 percent in the initial study period between last December and April, 2021.
However, when the ultra-contagious Delta variant became dominant, effectiveness of the two vaccines fell to 66 percent.
The report's authors said there were a number of possible explanations, including that protection from vaccines could be waning over time anyway, and the 66 percent estimate was based on a relatively short study period in which there were few infections.
"Although these interim findings suggest a moderate reduction in the effectiveness of Covid-19 vaccines in preventing infection, the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of Covid-19 vaccination," they said.
A number of studies have now concluded that vaccine efficiency has diminished in the face of Delta.
Protection against the most severe consequences of infection appears more stable, exceeding 90 percent, according to a recent CDC study of patients in New York.
Another CDC study, carried out earlier this year in Los Angeles, showed that unvaccinated people were 29 times more likely to be hospitalised with Covid-19 than the vaccinated.
Delta became the dominant strain in the United States in early July.
(with AFP)
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