- The Delta variant is the most common cause of new COVID-19 infections in the US, the UK, and elsewhere.
- Six studies suggest vaccines from Pfizer, Moderna, and AstraZeneca work against Delta, to varying degrees.
- These studies cover both symptomatic and asymptomatic COVID-19 infections.
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The highly-infectious Delta coronavirus variant has spread to at least 174 countries worldwide, from the US to Australia, causing a surge in COVID-19 cases. The variant has mutations that help it partially escape the immune response produced by vaccines.
The data for how well COVID-19 vaccines work against the Delta variant isn’t clear cut.
The vaccines may offer less protection against symptomatic illness caused by Delta compared to other variants.
Public Health England said in a report published on September 9 that, generally, COVID-19 vaccines were 79% effective against COVID-19 with symptoms caused by Delta after two doses.
Here’s how much protection COVID-19 vaccines give you against symptomatic Delta infections, based on the best available data from four studies.
UK study: Pfizer 88% effective, AstraZeneca 67%
A UK-based study from May, published in the New England Journal of Medicine on July 21, found that two doses of either AstraZeneca or Pfizer’s vaccine were highly effective against the Delta variant, from two weeks after the second dose.
- 33% effective after one dose, 88% effective after two.
- 33% effective after one dose, 67% effective after two.
Canadian study: Pfizer 87% effective after two doses, Moderna 72% after one dose
A Canadian study posted on Saturday, July 3, found that two doses of a COVID-19 vaccine worked as well against Delta as they did against the Alpha variant, which was previously dominant in the US.
The study hasn’t been scrutinized by experts in a peer-review.
- 56% effective from 14 days after first dose, 87% effective after two doses.
- 67% effective from 14 days after first dose. Not enough data for two doses.
- 72% effective from 14 days after first dose. Not enough data for two doses.
Israel study: Pfizer 64% effective after two doses
The Israel Ministry of Health said on Monday, July 6, that Pfizer’s vaccine worked slightly less well against the Delta variant than previous estimates. The number of people who got infected during the study period was small, and the figure may have included asymptomatic infections.
- 64% effective after two doses.
Scotland study: Pfizer 79% effective after two doses, 60% for AstraZeneca.
A Scotland-based study published as a letter to the Lancet medical journal on June 14 found that Pfizer’s vaccine offered “very good” protection against the Delta variant.
- 79% effective after two doses, at least 14 days after the second dose.
- 60% effective after two doses, at least 14 days after the second dose.
Studies that cover asymptomatic infections
Two real-world studies have looked at vaccine effectiveness against all COVID-19 infections, including those that didn’t cause symptoms.
Qatar study: Pfizer 53.5% effective after two doses, 84.8% for Moderna
A Qatari study posted on August 11 looked at vaccine effectiveness against any infection, including asymptomatic infections. It hasn’t been scrutinized by experts in a peer-review.
- 53.% effective after two doses, more than 14 days after the second dose.
- 84.8% effective after two doses, more than 14 days after the second dose.
UK study: Pfizer 80% effective after two doses, 67% for AstraZeneca
- 80% effective after two doses, at least 14 days after the second dose.
- 67% effective after two doses, at least 14 days after the second dose.
Why the numbers vary
It becomes harder to measure how well vaccines work in the real world compared with trials, because you can’t control who gets vaccinated and who doesn’t. Other differences between the two groups could influence the risk of getting sick from COVID-19. For example, those who chose not to get vaccinated could also be more likely to put themselves in risky situations that may expose them to the virus.
The numbers can also vary because they depend on other factors, including what you’re measuring, when you measure it, the age of the population you’re measuring it in, and whether people have had previous COVID-19 infections.
Stephen Evans, professor of medical statistics at the London School of Hygiene & Tropical Medicine, told Insider that in general, the more severe the illness caused by Delta, the better the COVID-19 vaccines appeared to work against it. But the evidence on vaccines’ effectiveness wasn’t strong, he said.