Does it matter that China’s Sinovac COVID vaccine is only 50% effective?

Mark Lynas

January 13, 2021

Researchers at Brazil’s Butantan Institute in Sao Paolo have announced that trials of the CoronaVac vaccine — made by the Chinese company Sinovac — show it is just over 50 percent effective overall, once very mild cases of the disease are taken into account, which is barely over the threshold needed for regulatory approval.

Last week the Institute reported the vaccine was 78 percent effective in mild cases and 100 percent effective against severe and moderate infections based on 250 COVID-19 cases among 13,000 volunteers among Brazil’s front-line health workers.

This seems to compare unfavorably with other COVID-19 vaccines — even though several countries, including Indonesia, Turkey and Singapore, have already placed orders for CoronaVac. To look more deeply at these figures, here’s a brief Q&A:

Is this 50 percent figure worrying?

At face value this 50 percent efficacy figure looks disappointing — it’s not much better than your average flu vaccine. But what might be more important is that the trial shows a 100 percent effectiveness in preventing severe COVID-19, of the sort that kills people. So if you want to save lives and stop health services from being overwhelmed, this vaccine is fine. If you want to eradicate COVID-19, maybe it’s not quite good enough.

The Sinovac vaccine is also being trialled in other countries isn’t it? What do those trials show so far?

Yes, it is also being trialled in Turkey and Indonesia. The trial group numbers in Turkey and Indonesia were much lower, however, less than a couple of thousand, and it’s impossible to draw any statistically significant conclusions from such small numbers.

How does this actually compare with the results for the other vaccines, e.g., the Pfizer-BioNTech and Oxford Astra-Zeneca vaccines?

Well, again on face value it’s not as good. But remember, these vaccines all use different platforms. The Chinese vaccine is an inactivated virus, basically a killed COVID virus, while the Pfizer vaccine uses mRNA and the Oxford/AstraZeneca uses a viral vector. The inactivated virus approach has been used for decades against many different diseases, so it is very well proven in the real world, but it maybe slightly weaker than the newer approaches.

What’s the explanation for these differences otherwise?

There are a couple of reasons why this might be somewhat unfair to Sinovac’s vaccine. One is that the levels of virus exposure among the trial participants, who were frontline health workers in Brazil, was very high. That means the vaccine had a tougher job to do to protect them. Another possibility is that the Chinese vaccine trial in Brazil may have been picking up much milder COVID cases who might not have registered as symptomatic in the Pfizer-BioNTech trial, for example. A third reason is that the two doses were only spaced 14 days apart. It now looks like if you wait much longer for the second dose you will likely get a better immune response and higher protection.

Are there other pros and cons that countries might need to take into account when comparing vaccines?

Yes, for example the mRNA vaccines look like they will be significantly more expensive and need deep freeze storage, so they’re harder to handle in countries without an advanced cold chain system. The Sinovac vaccine can just be kept in a fridge and should be much more stable to store. These things matter when you’re talking about immunizing a population not just in the tens or hundreds of millions, but in the billions.

Image by Shutterstock

Thanks to Lucy Bailey at the BBC World Service for the conversation that led to this piece.