Pfizer vaccine reliable, though not as effective as papers suggest

“Two Pfizer doses ‘are 99.96% effective’: Double jab weeks apart prevents almost ALL cases of Covid-19, research in Israel suggests”.

Daily Mail, 29 January 2021.

The Pfizer Covid-19 vaccine has been shown to be over 90% effective, but not necessarily as effective as recently claimed in the Daily Mail.

The Mail reported that the full two-dose vaccination programme was 99.96% effective. This was based on reports that among 715,425 people vaccinated in Israel, 317 (0.04%) later contracted the virus. 

The data was also reported by the Metro and the Sun, which suggested something similar by claiming the vaccine stopped or prevented 99.96% of cases.

This is not how to calculate how effective a vaccine is. This is normally done through what is called an efficacy rate. 

An efficacy rate is calculated by comparing the number of people who get infected after vaccination to the number who get infected in an unvaccinated control group.

So if you saw 90% fewer infections in the vaccinated group compared to the control group, you would say the vaccine has 90% efficacy.

What the Mail suggests is a measure of effectiveness is wrong, because it just looks at infections in the vaccinated group, not how that compares to a group who haven’t been vaccinated.

For example, the Oxford-AstraZeneca vaccine trial data showed that 30 people—or 0.5% of its vaccinated group—were later infected, compared to 101, or 1.7%, of the non-vaccinated group. This means the efficacy rate was 70.4% because infections were that much lower in the vaccinated group.

But if you looked simply at the rate of infection in each group separately, the vaccine would appear 99.5% effective (given only 0.5% of the vaccinated group later caught the virus), and not getting vaccinated would also be 98.3% effective, given only 1.7% of that group were later infected. 

This clearly demonstrates that you can’t just look at the infection rate among vaccinated people alone to calculate the efficacy rate.

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