- Millions of people around the world received a two-dose injection of the COVID-19 vaccine.
- The Pfizer and Moderna vaccines are probably 80% effective against symptomatic COVID-19 after a dose.
- A single injection of AstraZeneca 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 the two-dose COVID-19 vaccine.
The United States authorized vaccines from Moderna and Pfizer-BioNTech, while the United Kingdom authorized the injection of Pfizer, as well as one made by AstraZeneca and the University of Oxford. The United States has also authorized the Johnson & Johnson vaccine, which is in a single dose.
The UK is postponing the second dose of vaccines for up to 12 weeks to prioritize giving people the first vaccine. In the United States, the Center for Disease Control and Prevention recommended giving second doses of the Pfizer vaccine 21 days after the first and 28 days after the first to Moderna, with an interval of up to six weeks in “unavoidable” situations.
Data on vaccine performance after a dose is unclear – it depends on what you are measuring and when you are measuring it. Stephen Evans, professor of medical statistics at the London School of Hygiene and Tropical Medicine and a former member of the European Medicines Agency’s drug safety committee, helped Insider to break down the data.
Evans said the Food and Drug Administration’s presentation of final-stage test data was the best available. This is the amount of protection that an injection of each vaccine offers you, based on this data.
Pfizer-BioNTech: at least 80%
The Pfizer injection was 52.4% effective in protecting against COVID-19 with symptoms between the first and second doses, according to FDA documents. But the 52.4% figure includes the 11 days before the protection is activated after the first dose, so the actual percentage could well be higher.
The true value is between 29.5% and 84.5%, according to FDA documents. The variation was wide because few people caught COVID-19 on the test during that time.
Professor Stephen Evans.
Professor Stephen Evans
The Pfizer injection was 100% effective in protecting against hospitalization and death. However, this was based on a small number – only four people contracted severe COVID-19 in the study after receiving a placebo instead of the vaccine.
Evans said there is “very 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 you cannot be sure what happens after 21 days because it has not been fully tested.
Evans said this was based on his general reading of the trial data used by the FDA in its information document prior to authorization.
Modern: at least 80%
Moderna’s vaccine was 69.5% effective in preventing COVID-19 with symptoms between the first and the second dose, with a real value between 43.5% and 84.5%. The variation was quite wide because the number of people who took COVID-19 in the test during this period was low.
The figure of 69.5% includes the 13 days prior to the start of protection, so the actual percentage could be higher.
There were a small number of people in the Moderna trial – about 7% – who did not receive the second dose for unknown reasons. In this group, the injection was 50.8% effective in preventing COVID-19 with symptoms for up to 14 days after the first dose and 92.1% effective after 14 days.
It is not clear how well an injection of the vaccine protects against hospitalization and death because few people have contracted severe COVID-19 – two in the vaccine group and four in the placebo group.
Evans said you get at least 80% protection – and probably better than 90% – for the Modern vaccine against COVID-19 with symptoms after a single dose for 28 days. After 28 days, it was not clear why it had not been tested. Again, this was based on your overall reading of the FDA data, he said.
AstraZeneca: more than 70%
Evans said it is more difficult to determine a value for the AstraZeneca vaccine because the end-stage tests used different study designs, and a large study in the United States was underway.
The FDA also did not present the data for the injection in the same way that it did for other vaccines.
A single dose of the AstraZeneca injection was 76% effective in protecting against COVID-19 with symptoms for at least 90 days, according to data from the final-stage trial published in The Lancet on February 19. The study authors also reported that one dose provided 100% protection against hospitalization, but the numbers were small.
Based on his reading of existing studies, Evans said the effectiveness of a single dose for the AstraZeneca vaccine was probably at least 70% against COVID-19 with symptoms in the first 90 days. After this period of time, it is not clear, he said.
Johnson & Johnson: 66%
J&J analyzed protection against moderate to severe COVID-19 in trials, rather than symptomatic COVID-19, such as Pfizer, Moderna and AstraZeneca.
Protection started in 14 days and was 66.1% effective in 28 days. The vaccine’s effectiveness varied depending on the country in which it was used – it was 72% effective in the US, but 64% and 68% effective in South Africa and Brazil, respectively. Both countries have circulating coronavirus variants that can partially evade antibodies.
What percentage of effectiveness means
Percentage effectiveness of vaccines refers to the proportion of people who obtain full protection after a vaccine. With 80% effectiveness, 80% of people are fully protected and 20% are not.
For those who get full protection on the first try, the second injection improves the quality of the immune response and its durability.
For people who do not get full protection with the first injection, some will get full protection after the second dose. Some people will never get full protection from a vaccine because their immune system does not respond at all.