Efficacy of Covid-19 Vaccines: Definition, Details, and Reliabilities

Photo by Daniel Schludi on Unsplash

As of the time of writing, two Covid-19 vaccines have been granted emergency use in the US, which are developed by Pfizer-BioNTech and Moderna. In other places of the world, several other vaccines have also been used.

When discussing about different Covid-19 vaccines, the first thing mentioned is usually the efficacy. The so-called efficacy has become so well known that everyone knows a bit about it — at least the values. For the two vaccines in use in the US: the Pfizer-BioNTech vaccine has a 95% efficacy, and Moderna vaccine has a similar efficacy of 94.1%. The Oxford-AstraZeneca vaccine, will soon be approved in the US and is already in use in the UK and several other countries, reported an efficacy of 70.4%.

Definition of vaccine efficacy

These values all look high, and therefore, promising. But what do they really mean?

In fact, these values are calculated from the phase 3 clinical trials conducted by the vaccine developers. In phases 1 and 2 of the clinical trials, the safety of the vaccine is studied and the effectiveness is examined. The goal is to ensure that the vaccine is safe enough to apply to many people. The phase 3 trials are what define the efficacy. The phase 3 trials are larger, which involve tens of thousands of volunteers, supposedly having similar distributions of sex, age, and ethnicity as the whole population. These volunteers are randomly divided into two groups. For one group, the participants receive vaccines and for the other group, placebo (control product) are given to the participants. Since the participants don’t know whether they are given vaccines or not, they behave equally in their daily life, and are exposed to the same risks of the disease (here, Covid-19). After several months, the two groups both would have people infected with Covid-19. The efficacy is then calculated as:

So for a perfect vaccine, no one will be infected after vaccinated. The efficacy will then be 100%. The efficacy is thus the higher the better.

Now here is a short summary for the three major vaccines I mentioned earlier.

For the Pfizer-BioNTech vaccine, the Covid-19 cases in the vaccinated and unvaccinated groups are 8 vs 162 (after 7 days after the second dose), among the total 43548 participants. For the Moderna vaccine it’s 11 vs 185, and there are 30420 participants. As for the Oxford-AstraZeneca vaccine, it’s 30 vs 101 and 11636 participants.

Can vaccines stop transmission of Covid-19?

Still, details exist in these numbers.

In the trials of Pfizer-BioNTech and Moderna vaccines, only symptomatic infections are recorded, which means that the efficacy only indicates how well the vaccine can prevent people from developing symptoms, and it is unknown whether the vaccines can prevent the participants from getting infected with the virus or transmitting it to other people. As we know that asymptomatic patients can still transmit Covid-19, a vaccine which can also prevent asymptomatic infections will be even more beneficial for the whole society, as there are always people who do not or cannot receive vaccination.

The data revealed by Oxford-AstraZeneca gives some hints. In their trial, asymptomatic infections are recorded in a subset of participants. The number of asymptomatic infections in the vaccinated and unvaccinated groups are 29 vs 40, corresponding to an efficacy of 27.3%. Although the values are much lower than for the symptomatic infections, it is still something and can provide some extent of protection (And, it can possibly be higher — maybe you have already learned it somewhere else: when reducing the dose of the first dose (LD), the Oxford-AstraZeneca vaccine provides better protection. The efficacy is 90.0%. For asymptomatic infections, the efficacy is now 58.9%. But note that the the number of trial participants receiving LD is only 30% of those receiving standard dose, so the sample is smaller.)

Reliability of efficacy of different vaccines

For these three major vaccines, the data are published and the articles are peer-reviewed. Developers of other vaccines have also claimed different efficacies. However, some of these vaccine developers have not made the data publicly available (examples: the vaccines developed by Chinese pharmaceutical companies Sinovac and Sinopharm). As the data are not revealed and there is no publication in peer-reviewed journals, the efficacy may not be reliable. For example, if the participants in the phase 3 trial are biased toward younger population, then the claimed efficacy might not apply to elderly.

So, these are some of the important things you might want to know about vaccine efficacy. With these you should be able to understand better some of the controversies about different Covid-19 vaccines. For example, why are there different reports of efficacy on the Chinese vaccine Sinovac? The short answer is, the trials were carried out independently in several countries with different controlled conditions. Hence different results were produced.

Recently Novavax (on Jan. 28) and Johnson & Johnson (on Jan. 29) have also revealed their trial results and respective efficacy. The phase 3 trial results of Russia’s vaccine Sputnik V have also been published on the peer-reviewed journal The Lancet (on Feb. 2), which should help to ease the scepticism around its effectiveness. I hope this article helps you better interpret these values and the related issues.

See also
[1] Published paper of Pfizer-BioNTech vaccine
[2] Published paper of Moderna vaccine
[3] Published paper of Oxford-AstraZeneca vaccine
[4] How effective is a single vaccine dose against Covid-19?
[5] A side-by-side comparison of the Pfizer/BioNTech and Moderna vaccines
[6] Published paper of the Sputnik V vaccine




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Tzu-Hao Yang

Tzu-Hao Yang

Taiwanese living in Europe.

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