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Commentary: How ‘good’ does a COVID-19 vaccine need to be to stop the pandemic?

A vaccine needs to reach a certain level of efficacy before coronavirus restrictions can be eased, says an observer.

Commentary: How ‘good’ does a COVID-19 vaccine need to be to stop the pandemic?

A nurse wearing watches an ambulance driving away outside Elmhurst Hospital in the Queens borough of New York, US on Apr 20, 2020. (Photo: REUTERS/Lucas Jackson)

NEW YORK CITY: The US is pinning its hopes on a COVID-19 coronavirus vaccine, but will a vaccine alone be enough to stop the pandemic and allow life to return to normal?

The answer depends on a how “good” the vaccine ends up being.

In a study published on Jul 15 in the American Journal of Preventive Medicine, my colleagues and I used a computer simulation of every person in the country to show how effective a vaccine would have to be and how many people would have to get vaccinated to end the pandemic.

We found that a coronavirus vaccine’s effectiveness may have to be higher than 70 per cent or even 80 per cent before Americans can safely stop relying on social distancing.

By comparison, the measles vaccine has an efficacy of 95 per cent to 98 per cent, and the flu vaccine is 20 per cent to 60 per cent.

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That doesn’t mean a vaccine that offers less protection would be useless, but it would mean social distancing in some form may still be necessary.


Some political leaders have suggested that society will return to normal soon, especially if a vaccine becomes available by the end of the year or early in 2021. Some vaccines are currently in early-stage trials, but that timeline would still be very optimistic.

However, it is important to remember that a vaccine is like many other products: What matters is not just that the product is available but also how effective it is.

Small bottles labeled with a "Vaccine COVID-19" sticker and a medical syringe are seen in this illustration taken taken on Apr 10, 2020. (Photo: REUTERS/Dado Ruvic)

Different vaccines may offer different levels of protection. Scientists talk about this as the vaccine’s efficacy or effectiveness.

If 100 people who haven’t been exposed to the virus are given a vaccine that has an efficacy of 80 per cent, that means that on average 80 of them would not get infected.

The difference between efficacy and effectiveness is that the former applies when vaccination is given under controlled circumstances, like a clinical trial, and the latter is under “real-world” conditions. Typically, a vaccine’s effectiveness tends to be lower than its efficacy.

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Since COVID-19 coronavirus vaccines are still under development, now is the time to set vaccine efficacy levels to aim for, as well as to manage expectations. Running computer simulations is really the only way to ethically do this.

Typically, in an epidemic or pandemic, as more people are exposed to the virus, the number of new infections per day steadily increases until it reaches a peak and begins to drop. Of course, how long this takes depends upon how the virus and the response to it may evolve over time.

To stop the pandemic, the number of new infections per day needs to drop to zero, or at least to a very low number, as quickly as possible.

If the COVID-19 pandemic were just beginning and the population infected was close to 0 per cent, the simulations show that vaccine efficacy would have to be at least 60 per cent to stop the coronavirus if the entire population was vaccinated.

Given the number of susceptible people who couldn’t be vaccinated because of age or health problems and the number who would refuse to be vaccinated, that’s probably impossible.

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(Graphic: Bruce Y Lee)

If only 75 per cent of the population gets vaccinated, the vaccine efficacy would have to be around 70 per cent. If only 60 per cent of people get vaccinated, the threshold goes even higher, to around 80 per cent. It’s all about making sure the virus can’t find more people to infect.

Those numbers assume that a person infected with the virus infects 2.5 other people on average. If the virus is more contagious, the vaccine has to be more efficient.

Now, the further along the pandemic is, the less the height of the peak can be reduced. It’s like climbing a mountain – you are already at a certain height. Plus, it is harder to shut a pandemic down when there are more infectious people running around.

So, when 5 per cent of the population has already been infected with the virus, the best that you can do is reduce the peak by around 85 per cent. The difference between 0 per cent and 5 per cent can add up to millions of infections.

So far, about 1 per cent of the US population has been confirmed to have been infected, but officials estimate the actual percentage is much higher.

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(Graphic: Bruce Y Lee)
(Graphic: Bruce Y Lee)


Based on these findings, a vaccine with an efficacy as low as 60 per cent could still stop the pandemic and allow society to return to normal. However, most if not all of the population would have to be vaccinated.

This seems unlikely, given polls showing that only about three-quarters of Americans say they would get a coronavirus vaccine if assured that it was safe.

With fewer people protected, a vaccine would have to have an efficacy of at least 80 per cent to be able to stop the pandemic by itself, meaning social distancing could be completely relaxed. This can provide a target to aim for when developing COVID-19 coronavirus vaccines.

Again, all of this doesn’t mean that a vaccine with a lower efficacy would not be useful. It would mean that social distancing and mask-wearing likely would have to continue until the pandemic runs its course or a vaccine that is actually “good enough” arrives.

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Bruce Y Lee is Professor of Health Policy and Management at the City University of New York. This commentary first appeared on The Conversation.

Source: CNA/el


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