More than 179 million Americans and more than 44 million Britons have received their first dose of a two-shot COVID-19 vaccine.

The US has authorized vaccines from Moderna and Pfizer-BioNTech, while the UK has authorized Pfizer's shot as well as one made by AstraZeneca and Oxford University. Both countries have authorized Johnson & Johnson's vaccine, which is a single dose.

The UK is delaying the second dose of the vaccines for up to 12 weeks for most people to prioritize giving people their first shot because of an initial shortage of vaccines. In the US, the Centers for Disease Control and Prevention has recommended giving second doses 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 how well the vaccines work after one dose isn't clear cut – it depends on what you're measuring, and when you're measuring it. 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.

Evans said the Food and Drug Administration presentation of the data from late-stage trials of each vaccine was generally the best data available. This is how much protection one shot of each vaccine gives you, based on that data.

Pfizer-BioNTech: at least 80 percent

Pfizer's shot was 52.4 percent effective at protecting against COVID-19 with symptoms between the first and second dose, according to the FDA documents. But the 52.4 percent figure includes the 11 days before protection kicks in after the first dose, so the real percentage could well be higher.

The true value lies between 29.5 percent and 84.5 percent, according to the FDA documents. There was a wide range because not many people caught COVID-19 in the trial during this time period.

Pfizer's shot was 100 percent effective at protecting against hospitalization and death. This was based on a small number though – only four people got severe COVID-19 in the trial after receiving placebo rather than the vaccine.

Evans said there was "pretty clear evidence" that you get at least 80 percent protection – and "probably" better than 90 percent – for Pfizer's vaccine against COVID-19 with symptoms after a single dose. He said you couldn't be absolutely sure what happens after 21 days because it hadn't been fully tested.

Evans said this was based on his overall reading of the trial data used by the FDA in their briefing document before authorization.

Moderna: at least 80 percent

Moderna's vaccine was 69.5 percent effective at preventing COVID-19 with symptoms between the first and second dose, with a true value between 43.5 percent and 84.5 percent. There was a fairly wide range because the number of people that caught COVID-19 in the trial during this time period was low.

The 69.5 percent figure includes the 13 days before protection starts, so the real percentage could be higher.

There were a small number of people in Moderna's trial – about 7 percent – that didn't get their second dose for unknown reasons. In this group, the shot was 50.8 percent effective at preventing COVID-19 with symptoms for up to 14 days after the first dose and 92.1 percent effective after 14 days.

It is unclear how well one shot of the vaccine protects against hospitalization and death because not many people got severe COVID-19 – two in the vaccine group and four in placebo.

Evans said that you get at least 80 percent protection – and probably better than 90 percent – for Moderna's vaccine against COVID-19 with symptoms after a single dose for 28 days. After 28 days it was unclear because it hadn't been tested. Again, this was based on his overall reading of the FDA data, he said.

AstraZeneca: more than 70 percent

Evans said it was harder to ascertain a figure for AstraZeneca's vaccine because late-stage trials used differing study designs, and a large US study was ongoing. The FDA also has not yet presented the data for the shot in the same way it has done for other vaccines.

A single dose of AstraZeneca's shot was 76 percent effective at protecting against COVID-19 with symptoms for at least 90 days, according to late-stage-trial data published in The Lancet on February 19. The study authors also reported that one dose provided 100 percent 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 probably at least 70 percent against COVID-19 with symptoms for the first 90 days. After this time period, it's unclear, he said.

Johnson & Johnson: 66 percent

J&J looked at protection against moderate to severe COVID-19 in trials, rather than symptomatic COVID-19, like Pfizer, Moderna, and AstraZeneca.

Protection kicked in at 14 days and was 66.1 percent effective at 28 days. The vaccine's efficacy varied depending on the country it was used in – it was 72 percent effective in the US but 64 percent and 68 percent effective in South Africa and Brazil, respectively. These countries both have coronavirus variants circulating that could partially evade antibodies.

What percentage efficacy means

Percentage efficacy for vaccines refers to the proportion of people that get full protection after a vaccine. With 80 percent efficacy, 80 percent of people have full protection, and 20 percent 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.

The latest real-world data: One shot significantly reduces infections and transmission

  • A UK study found Pfizer or AstraZeneca's vaccine cut COVID-19 infections with symptoms by 72 percent after one dose, and protection probably held up for 10 weeks. Protection from Pfizer's vaccine rose to 90 percent 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 percent effective against all coronavirus infections from 14 days.

  • A Scottish study found that a single dose of Pfizer's vaccine was 91 percent effective against hospitalization at 28 to 34 days following vaccination. One dose of AstraZeneca's vaccine was 88 percent 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 percent.

  • A South Korean study found one dose of Pfizer's vaccine was 89.7 percent effective at preventing COVID-19 in South Koreans aged over 60, at least two weeks after vaccination. AstraZeneca's vaccine was 86 percent 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 percent 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.

  • An Israel study showed that Pfizer's vaccine was 54 percent effective against symptomatic COVID-19, from 13 days to 24 days after vaccination, a figure comparable to the late stage trial data presented to the FDA.

  • A UK study estimated that a single dose of either Pfizer or AstraZeneca's vaccine was between 56 percent and 62 percent effective at preventing COVID-19 infection caused by the Alpha variant in people over 75 years-old, four to seven weeks after the first dose. The severity of illness that the shots protected against was unclear, but probably included asymptomatic infections.

  • A UK study estimated that one dose of Pfizer vaccine was 79.3 percent effective at reducing the risk of hospitalization from COVID-19 in people aged over 80. A single shot of AstraZeneca's was 80.4 percent effective, the researchers said.

Newest data suggests second shot provides better protection against variants

Real-word data from the UK posted May 23 by Public Health England showed that Pfizer's and AstraZeneca's COVID-19 vaccines worked better against the variants when two doses were given rather than just one. Both vaccines were 30 percent effective against COVID-19 with symptoms caused by the Delta variant, first identified in India, three weeks after the first dose.

This was boosted to between 60 percent and 88 percent effectiveness two weeks after the second dose. The two vaccines were 50 percent effective against COVID-19 with symptoms against the variant first found in the UK, Alpha, three weeks after the first dose. This increased to between 66 percent and 93 percent two weeks after the second dose.

Dr. Anthony Fauci, President Joe Biden's chief medical advisor, said on June 8 that getting two doses of COVID-19 vaccines would stop the Delta variant from spreading across the US.

In the UK, Professor Deborah Dunn-Walters, chair of the British Society for Immunology COVID-19 Taskforce, said in a statement on June 4 that two doses of Pfizer's vaccine were "critical for protection" against emerging strains of the virus.

This article was originally published by Business Insider.

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