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Pfizer's COVID-19 vaccine works very well against the Delta variant - but only after 2 doses

Pfizer's COVID-19 vaccine works very well against the Delta variant - but only after 2 doses
Pfizer's COVID-19 vaccine works very well against the Delta variant - but only after 2 doses
The study on 19,000 people in the UK found one Pfizer shot was about 36% effective against Delta, but it was highly effective after two shots.
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  • A UK study suggests two-dose Pfizer and AstraZeneca vaccines work well against Delta infections.
  • After one shot, the vaccines were only 30% to 36% effective against symptomatic Delta infections.
  • But with two shots, Pfizer's and AstraZeneca's vaccines were 88% and 67% effective, respectively.
  • See more stories on Insider's business page.

A study of more than 19,000 teens and adults across the UK provides some of the first compelling, peer-reviewed, and large-scale evidence that you need both doses of two-dose vaccines to get good protection from the Delta variant.

The findings from Public Health England suggest that vaccine protection against Delta is very strong when people get both shots.

But the research, published in The New England Journal of Medicine on Wednesday, showed that just one dose of either Pfizer's or AstraZeneca's COVID-19 vaccine protected people from symptomatic infections only about one-third of the time.

"It's clear how important the second dose is to secure the strongest possible protection against COVID-19 and its variants," Matt Hancock, the UK's secretary of state for health and social care, said when an unreviewed preprint of the study was released in May.

When patients were fully vaccinated, with two shots both given at least two weeks to take effect, Pfizer's COVID-19 vaccine became about 88% effective against symptomatic COVID-19 from the Delta variant, while AstraZeneca's vaccine was 67% effective against it.

That is almost as good as those vaccines performed in clinical trials before the new variant was detected, but it does suggest that breakthrough infections in vaccinated people will become slightly more common now that Delta is here.

One shot was only about 30% effective against COVID-19 symptoms with Delta

man staring at the camera, about to get COVID-19 shot administered by nurse
A man gets his second vaccine dose in Bridgeport, Connecticut, on April 20. Joseph Prezioso / AFP via Getty Images

This study was possible because the UK maintains a national vaccination register and the proportion of positive COVID-19 cases that are sequenced in the UK is far higher than in the US (about 60% as of May).

In their comparison of a patient's vaccination status, their COVID-19 test results, and variant sequencing to determine whether their infection was caused by the Alpha (B.1.1.7) variant or Delta, researchers were able to determine how well the Delta variant evaded vaccine protection.

With only one shot on board, Pfizer's vaccine was just 36% effective against symptomatic Delta cases, while AstraZeneca's vaccine was 30% effective, the researchers found.

Other research suggests that the Delta variant is more contagious, and that may be, in part, because people who are infected tend to shed more virus while they are sick.

The researchers did not estimate vaccine effectiveness for severe disease and death in this study, but other research (and hospital data) suggests that COVID-19 vaccines are still extremely good at keeping people alive and out of hospital beds.

Vaccinated people are starting to report getting sick more often now that Delta is around, but their infections are generally milder, with common symptoms including headaches, runny noses, congestion, and sore throats.

Getting two shots of a vaccine gives the body a chance to develop a more robust immune response to COVID-19, which ramps up its attack on the virus.

"That's the reason why the second dose is much more reactive," professor Akiko Iwasaki, who studies viruses at Yale, previously told Insider. "This is a sign that your immune system is working because you develop a much worse response the second time, based on these antibodies and T cells that are detecting the viral antigen and attacking your own cells."

Even with Delta prevalent, all the usual pandemic protocols, including masking, distancing, and limiting exposure, still help.

"Even modest mask use combined with vaccination can really put the brakes on even the Delta variant," Christopher Murray, the director of the Institute for Health Metrics and Evaluation, previously told Insider.

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