• Millions of people worldwide have had one shot of two-dose COVID-19 vaccines.
• Pfizer and Moderna vaccines are likely 80% effective against symptomatic COVID-19 after one dose.
• A single AstraZeneca shot is probably at least 70% effective at preventing symptomatic COVID-19. //

Stephen Evans, a professor of medical statistics at the London School of Hygiene & Tropical Medicine and a former drug-safety committee member at the European Medicines Agency, helped Insider break down the data. //

Percentage efficacy for vaccines refers to the proportion of people that get full protection after a vaccine. With 80% efficacy, 80% of people have full protection, and 20% don't.

For those who get full protection the first time around, the second shot improves the quality of the immune response and its durability.

For the people who don't get full protection with the first shot, some will get full protection after the second dose. Some people won't ever get full protection from a vaccine because their immune system doesn't respond at all. //

A UK study found Pfizer or AstraZeneca's vaccine cut COVID-19 infections with symptoms by 72% after one dose, and protection probably held up for 10 weeks. Protection from Pfizer's vaccine rose to 90% after two doses. The study hasn't been peer-reviewed.
A US study of essential workers found that a single dose of Pfizer of Moderna's COVID-19 vaccines were 80% effective against all coronavirus infections from 14 days.
A Scottish study found that a single dose of Pfizer's vaccine was 91% effective against hospitalization at 28 to 34 days following vaccination. One dose of AstraZeneca's vaccine was 88% effective against hospital admissions after the same time period.
A UK study found that a single dose of either Pfizer or AstraZeneca's vaccine cut spread of symptomatic COVID-19 within a household by up to 50%.
A South Korean study found one dose of Pfizer's vaccine was 89.7% effective at preventing COVID-19 in South Koreans aged over 60, at least two weeks after vaccination. AstraZeneca's vaccine was 86% effective at preventing COVID-19 after one dose. The severity of illness that the shots protected against was unclear — generally they're more effective at preventing COVID-19 infections that caused hospitalization or death.
An English study found that a single dose of either Pfizer or AstraZeneca's vaccine was about 80% effective at preventing hospitalization in people over 70-years-old. Protection lasted for at least 6 weeks, including against the Alpha variant first identified in the UK.