TravelDream
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- 7 Aug 2016
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Yes, plenty of research out there, too much to quote. And efficacy varies by what it is preventing - infection, symptomatic disease, hospitalisation, death. And it varies by age group. And vaccine type. But very very broadly, one dose gives something between 60-80% efficacy, 2 doses 80-99%.
This is true, but isn't quite the full story.
There are a lot of numbers put out there on efficacy and people often pick and choose their favourite numbers. First, as you say, what is efficacy? Is it symptomatic infection? Death? Asymptomatic infection? Hospitalisation? Transmission? They are all different and could give radically different numbers.
Generally, most studies talk about 'symptomatic infection' which means the people they refer to have symptoms and then test positive for Covid. Most don't look at asymptomatic Covid where people are Covid positive, but have no symptoms - though a couple have.
First, the phase three trials. They are highly controlled studies and they all talk about symptomatic infection. That means people received two doses of the vaccine (some real and some placebos). Then they were asked to take Covid tests if they had any symptoms of infection. Those who had the vaccine and those who had the placebo were then compared. Headline numbers were:
Pfizer: 95% https://www.pfizer.com/news/press-r...iontech-announce-publication-results-landmark
Moderna: 94.1% https://investors.modernatx.com/new...-primary-efficacy-analysis-phase-3-cove-study
Oxford/ AZ: 70.4% https://www.astrazeneca.com/media-c...analysis-results-published-in-the-lancet.html
There are lots of numbers in these articles. The only true comparable though is the headline figure.
All three provided 100% protection against death and 'serious infection', but serious infection was defined differently by each company so, again, is not totally comparable.
The phase three trials did NOT check for efficacy after one dose.
There have then been many 'real-world' studies. These are not as controlled as phase three trials, but the large numbers involved make them pretty accurate.
These numbers have generally been even better than the phase three trials and several have looked at single-doses.
- Like this one in the BMJ from the UK. It showed that a single dose of Oxford-AZ was up to 73% effective against symptomatic infection whilst the Pfizer vaccine was up to 61% effective. https://www.bmj.com/content/373/bmj.n1088
- Or this study from the Korean Government. It showed that a single dose of Oxford-AZ in the over 60s was up to 86.0% effective against symptomatic infection whilst the Pfizer vaccine was up to 89.7% effective.
S.Korea says AstraZeneca, Pfizer COVID-19 vaccines 87% effective after first shot
One dose of COVID-19 vaccines from AstraZeneca Plc (AZN.L) and Pfizer (PFE.N) was 86.6% effective in preventing infections among people aged 60 and older, real world data released by South Korea showed on Wednesday.
www.reuters.com
Why the different numbers? Well, first, the numbers aren't that different and all show the vaccines are highly effective. There are a whole range of factors involved which mean no two studies are likely to give the exact same numbers. In layman's terms, these real-world studies could almost be described as carefully constructed and very educated back of a fag packet type stuff.
The real worry with this new Indian variant is that it appears to make vaccines a little less effective, especially in relation to first doses.
This data comes from Public Health England and was published in the BMJ.
Do note though this is a preprint paper meaning it is preliminary data, hasn't be subject to peer review and is subject to change.
B.1.617.2 is the Indian variant and B.1.1.7. the Kent variant.
A preprint paper released by Public Health England on 22 May showed that between 5 April and 16 May the Pfizer vaccine was 88% effective, two weeks after the second dose, against the B.1.617.2 variant and 93% against B.1.1.7, known as the UK or Kent variant.1 The AstraZeneca vaccine was 60% effective against B.1.617.2 at two weeks after the second dose and 66% against the Kent variant.
But both vaccines were only 33% effective against symptomatic disease from B.1.617.2 three weeks after the first dose, whereas they were 50% effective against B.1.1.7.
Covid-19: Single vaccine dose is 33% effective against variant from India, data show
The UK government has been urged to speed up giving two doses of the covid-19 vaccine after data showed a single dose was only 33% effective against the B.1.617.2 variant first detected in India, which continues to see a rapid growth in cases in the UK. A preprint paper released by Public...
www.bmj.com
The worry about the single-dose strategy is why the government has started to bring forward second doses among the elderly and most vulnerable.