- Millions of people worldwide have had one shot of COVID-19 two-dose vaccines.
- Pfizer and Moderna vaccines are likely to be 80% effective against symptomatic COVID-19 after a single dose.
- A single AstraZeneca shot is probably at least 70% effective in preventing symptomatic COVID-19.
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More than 60 million Americans and more than 22 million Britons received their first dose of COVID-19 vaccine with two shots.
The US has approved vaccines from Moderna and Pfizer-BioNTech, while the UK has approved Pfizer’s approval as well as one made by AstraZeneca and Oxford University. The US has also approved the Johnson & Johnson vaccine, which is a single dose.
The UK is delaying the second dose of vaccines to 12 weeks to give priority to giving people their first chance. In the US, the Centers for Disease Control and Prevention recommended giving the second dose of Pfizer’s vaccine 21 days after the first and 28 days after the first for Moderna, with an interval of up to six weeks in ‘unavoidable’ situations.
The data for the action of the vaccines after one dose are not clear – it depends on what you measure and when you measure it. Stephen Evans, a professor of medical statistics at the London School of Hygiene & Tropical Medicine and a former member of the Medicines Safety Agency at the European Medicines Agency, helped Insider break down the data.
Evans said presenting the data from the late-stage trials to the Food and Drug Administration was the best data. This is how much protection one shot of each vaccine provides, based on the data.
Pfizer-BioNTech: at least 80%
According to FDA documents, Pfizer’s shot was 52.4% effective against COVID-19 with symptoms between the first and second dose. But the figure of 52.4% contains the 11 days before the protection starts after the first dose, so the actual percentage may be higher.
The true value, according to FDA documents, is between 29.5% and 84.5%. There was a wide variety because not many people caught COVID-19 in trial during this period.
Professor Stephen Evans.
Professor Stephen Evans
Pfizer’s shot was 100% effective in protecting against hospitalization and death. However, it was based on a small number – only four people got severe COVID-19 in the trial after receiving placebo rather than the vaccine.
Evans said there is ‘fairly clear evidence’ that you get at least 80% protection – and ‘probably’ better than 90% – for Pfizer’s COVID-19 vaccine with symptoms after a single dose. He said that after 21 days you can not be completely sure because it has not been fully tested.
Evans said it was based on his general reading of the trial data the FDA used in their pre-authorization information document.
Modern: at least 80%
The Moderna vaccine was 69.5% effective in preventing COVID-19 with symptoms between the first and second dose, with an actual value between 43.5% and 84.5%. There was a fairly wide range because the number of people who caught COVID-19 in trial during this period was low.
The 69.5% figure includes the 13 days before protection, so the actual percentage may be higher.
There were a small number of people in Moderna’s trial – about 7% – who for unknown reasons did not receive their second dose. In this group, the shot was 50.8% effective in preventing COVID-19 with symptoms up to 14 days after the first dose and 92.1% effective after 14 days.
It is unclear how well one shot of the vaccine protects against hospitalization and death, as not many people have received severe COVID-19 – two in the vaccine group and four in placebo.
Evans said you get at least 80% protection – and probably better than 90% – for Moderna’s vaccine against COVID-19 with symptoms after a single dose for 28 days. After 28 days, it was unclear because it had not been tested. Again, this was based on his general reading of the FDA data, he said.
AstraZeneca: more than 70%
Evans said it was more difficult to determine a figure for the AstraZeneca vaccine because trials used different study designs in the late stages, and a large U.S. study was underway.
The FDA has also not yet provided the data for the shot, as it has for other vaccines.
A single dose of AstraZeneca’s shot was 76% effective in protecting COVID-19 for at least 90 days with symptoms, according to data in the late-stage study published in The Lancet on 19 February. protection against hospitalization, but the numbers were small.
Based on his reading of existing studies, Evans said the single-dose efficacy for AstraZeneca’s vaccine was likely to be at least 70% versus COVID-19 in the first 90 days. After this period, it is unclear, he said.
Johnson & Johnson: 66%
J&J looked at protection against moderate to severe COVID-19 in trials, rather than symptomatic COVID-19, such as Pfizer, Moderna and AstraZeneca.
Protection started at 14 days and was 66.1% effective at 28 days. The effectiveness of the vaccine depended on the country in which it was used – it was 72% effective in the USA, but 64% and 68% effective in South Africa and Brazil respectively. These countries both have coronavirus variants that can partially elude the antibodies.
What percentage does efficiency mean?
The percentage of effectiveness for vaccines refers to the percentage of people who receive full protection after a vaccine. With an efficiency of 80%, 80% of the people have full protection and 20% do not.
For those who get full protection for the first time, the second shot improves the quality of the immune response and its durability.
For people who do not get full protection with the first dose, some will get full protection after the second dose. Some people will never get full protection against a vaccine because their immune system does not respond at all.