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A study of more than 19,000 teens and adults across the UK provides some of the first compelling, peer-reviewed, large-scale evidence that you really need both doses of two-dose vaccines in order to get good protection from the Delta variant.
The findings from Public Health England suggest that vaccine protection against Delta is still very strong when people get both shots.
However, the research, published in the New England Journal of Medicine on Wednesday, showed that just one dose of either Pfizer or AstraZeneca’s COVID-19 vaccine only protected people from symptomatic infections about a 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,” UK Health and Social Care Secretary Matt Hancock said when an unreviewed preprint of the study was released back 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 with Delta, 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
This study was possible because the UK maintains a national vaccination register, and because the proportion of positive COVID-19 cases that are sequenced in the UK is far higher than in the US (around 60%, as of May 2021).
Comparing a patients’ 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.
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.
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Giving two shots of a vaccine gives the body a chance to develop more robust immune response to COVID-19, ramping 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 of 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.