A statistician explains: What does ‘90% efficacy’ for a Covid-19 vaccine mean?
The data we have so far only covers short-term efficacy, and the numbers may change in the coming months.
Vaccines can be very effective at stopping diseases being spread however lets keep in mind what this means and continue to test other medicines to use as well because we dont know how long the vaccine is effective
The Oxford-AstraZeneca COVID-19 vaccine may be able to reduce transmission of the coronavirus, while also stopping serious disease and death from COVID-19, early data suggests.
the researchers at the University of Oxford conducted weekly coronavirus tests of participants in the U.K. enrolled in a vaccine trial and found that the rate of positive results declined by about 67% after participants received one dose. Testing negative means no virus is present and makes it less likely a person is infected, even asymptomatically. People without detectable virus in their respiratory tract can't spread the virus. The work has not yet been peer-reviewed but was published as a preprint with the Journal The Lancet.
Other vaccine studies, by contrast, have mainly tested people who showed symptoms of COVID-19, not everyone enrolled in the trial, which means they couldn't tell how many vaccinated people were asymptomatic, but still infectious to others.
There’s abundant evidence that people can get infected with the coronavirus without ever showing symptoms. And so it’s possible that a number of people who got vaccinated in the clinical trials got infected, too, without ever realizing it. If those cases indeed exist, none of them are reflected in the 95 percent and other efficacy rates quoted.
We should also keep in mind that vaccine efficacy is just a statistical measurement of short-term performance of the vaccine. It cannot guarantee the durability of the vaccine beyond the time frame of experimentation, which is only a few months in these cases.
It is still not very clear how long a vaccine would provide protection against the disease. Whatever be the case, vaccine efficacy might drastically change if the study individuals are monitored for another few months, at least
A 90% efficacy (ie 10% RR) means that the proportion of infection in the vaccinated group is about one-tenth of the proportion of infection in the placebo group
The Moderna trial involved 30,000 people, and more than 43,538 people have been enrolled in the Pfizer/BioNTech vaccine trial – approximately half receiving the vaccine, and the remaining a “placebo”, in each trial. Of the first 95 to develop Covid-19 symptoms in the Moderna trial, only five were in the vaccine group, and the remaining 90 were in the placebo group.
Thus, the RR for Moderna vaccine up to this point of time is 5/90, and vaccine efficacy is (1 – 5/90), which is 94.44%.
For example, if among the next 100 infections 98 are from placebo group and 2 from the vaccine group, the efficacy of Moderna vaccine would be enhanced to
[1 – (5+2)/(90+95)], i.e. 96.28%. However, if the vaccine effect wanes in course of time and 70 and 30 of the next 100 infections are from the placebo and vaccine groups, respectively, the overall efficacy would become [1 – (5+30)/(90+70)], i.e. 78.13%. However, if the next 100 infections occur equally within the two groups, the resultant efficacy will be only 60.71%
The data we have so far only covers short-term efficacy, and the numbers may change in the coming months.
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You might assume that 95 out of every 100 people vaccinated will be protected from Covid-19. But that’s not how the math works.
www.nytimes.com
This is the first time that a coronavirus vaccine has shown it may reduce the spread of the virus.
www.livescience.com
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