The efficacy rate and what it really means

Edson Guido, ABS-CBN Data Analytics

Posted at Dec 28 2020 03:42 PM | Updated as of Dec 28 2020 03:56 PM

The efficacy rate and what it really means 1
A man works in the packaging facility of Chinese vaccine maker Sinovac Biotech, developing an experimental coronavirus disease (COVID-19) vaccine, during a government-organized media tour in Beijing, China, Sept. 24, 2020. Thomas Peter, Reuters/File

The 50% efficacy rate of the Sinovac vaccine from China has made headlines recently.

For the Philippines' Department of Science and Technology, the 50% efficacy is "acceptable" given that is the minimum requirement set by the World Health Organization. The Department of Health also agreed, reassuring Filipinos that the Chinese vaccine indeed meets international standards.

Still, many have expressed concern as the reported efficacy of Sinovac lags behind the 95% efficacy rate of Pfizer and the 94% of Moderna's.

So, what does 50% actually mean? How does it translate to real life? Let’s do the math.

For instance, 50,000 participants in a clinical trial were split evenly between the vaccine and placebo groups. Of the 300 COVID-19 cases that were found, 200 were in the placebo group and only 100 were in the vaccine group. Mathematically, the efficacy rate is then equivalent to a 50% risk reduction. In this particular example, getting a vaccine reduced the infection risk to half of those unvaccinated.

So here is what the efficacy rate means — and what it doesn’t mean.

A 50% efficacy rate indicates that a person is 50% less likely to become infected relative to someone who didn’t get the vaccine.

This does not mean that half of those who took the vaccine won’t get COVID-19 and the other half will. The efficacy rate is always relative to the risk of those who are exposed to the virus.

Because of this, it's important to know the infection risk for those who didn't get the vaccine. If we use the positivity rate of 5% as a proxy (the positivity rate of the Philippines in the last two weeks), then the risk of getting COVID-19 for those who took a vaccine of 50% efficacy is reduced to 2.5%. That is how the math works.

However, it’s important to note that the efficacy rates were derived from clinical trials that happened under a controlled environment. These conditions may or may not be met in the real world. Hence, it’s possible that there will be a difference between the reported efficacy rates and the effectiveness of the vaccines.

Pfizer and Moderna vaccines need to be stored in lower temperatures to maintain their efficacy and the cold chain infrastructure will be a logistical challenge in some areas. Furthermore, with most COVID-19 vaccines requiring two doses, previous studies have shown that not everyone comes back for the second dose. These are just some of the hurdles that may potentially change efficacy rates in the real world.

To sum, a 50% efficacy rate doesn’t mean that the vaccine only works half of the time. Rather, this rate means the vaccine halved the chances of people who got the vaccine from becoming sick relative to those who didn’t get the vaccine.

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