COVID-19 vaccine efficacy against Delta variant reduces over time –Study


Angela Onwuzoo

Researchers at the University of Oxford, United Kingdom, have found that Pfizer–BioNTech and Oxford–AstraZeneca COVID-19 vaccines are effective against the highly infectious Delta variant of SARS-CoV-2 — but their protection drops away over time.

According to them, the vaccines’ effectiveness fell to 90 per cent, 85 per cent and 78 per cent after 30, 60 and 90 days, respectively.

The team compared the effectiveness of shots made by AstraZeneca, Pfizer and Moderna vaccines against Alpha and Delta variants, the two most recent dominant strains in the US and UK, using test results from nearly 750,000 people in the U.K.’s COVID-19 household infection survey.

While the vaccines showed strong protection against COVID-19, the study, which has not yet been published or peer-reviewed, found they were less effective against the Delta variant. 

The Moderna vaccine appeared to be the most effective against Delta after one shot, the researchers wrote, though the group said the young average age of recipients could be responsible for this finding and that a full assessment of two Moderna shots was not possible in the study.

The researchers found that while the Pfizer shot was initially much more protective than the AstraZeneca vaccine, this waned rapidly after vaccination and in four or five months matched the more stable level of protection offered by AstraZeneca.  

Despite diminished protection, the researchers said that getting fully vaccinated against COVID-19 remains the best way to protect against the Delta variant and that the “effectiveness of two doses remains at least as great as a protection afforded by prior natural infection.”

The drop in effectiveness shouldn’t be cause for alarm, says Sarah Walker, a medical statistician at the University of Oxford who led the study. 

“Both of these vaccines, two doses are still doing really well against Delta.

“Most of our tests are monthly; we can’t really say very much at all about how long people are infectious for and particularly whether that’s different with Delta.

“Anyone who thinks that if they get infected having been vaccinated, they can’t transmit — that isn’t likely to be true.”

The study shows that vaccinated people who become infected with the Delta variant carry high peak levels of the virus. 

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