May 15, 2021

How effective should the coronavirus vaccine be in curbing the pandemic?


The United States has pinned its hopes on the discovery of a COVID-19 vaccine, but would a vaccine alone be enough to curb the pandemic and allow life to return to normal?

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

In a Article published on July 15 in the magazine American Journal of Preventive MedicineMy colleagues and I used a computer simulation based on the entire population of the United States to see how far a vaccine would have to be effective, and how many people would have to be vaccinated, to end the pandemic.

We found that the efficacy of the coronavirus vaccine would have to be greater than 70%, and even as much as 80%, for Americans to safely leave behind current social distancing measures. In comparison, the measles vaccine is between 95% and 98% effective. The flu, between 20% and 60%.

This does not mean that a vaccine that offers less protection is not useful, but that certain measures of social distancing will continue to be necessary.

What is the “efficacy” of a vaccine?

Some political leaders have claimed that countries will soon return to normal, especially if a vaccine is available. at the end of the year or at the beginning of 2021. Although it is true that currently there are some vaccines in the preliminary trial phase, these deadlines are very optimistic.

However, it is important to remember that a vaccine is like many other products: the important thing is not only to have them, but also to what extent they are effective. Let’s take clothing as an example. If you go to a formal dinner, the underwear covers your body only partially, much less than the situation would require … But that does not mean that it is useless.

Similarly, different vaccines may offer different levels of protection. Scientists refer to this either as the effectiveness or as the effectiveness of a vaccine.

If we provide 100 people who have not been infected with a vaccine whose efficacy is 80%, that means that, on average, 80 of them will not be infected.

The difference between efficacy and effectiveness is that the former refers to when the vaccine is applied under well-controlled conditions, such as a clinical trial, while the latter is used when supplied under real-world conditions.

Typically, the effectiveness of a vaccine is less than its effectiveness.

Simulations show what could happen

As coronavirus vaccines are still under development, at this time we need to set our goals in terms of efficacy and know how to manage expectations. The only way to do both ethically is through computer simulations.

For this study, our team from the PHICOR project of the City University of New York, in collaboration with scientists from the National School of Tropical Medicine from the Baylor School of Medicine (both in the US), has developed a computer simulation model that reproduces the social interactions of the entire population of the United States.

Thanks to this model we were able to simulate different levels of contagion among this virtual population and also to project what its expansion would be like in various pandemic scenarios. In each of them, the chances that an infected person ended up needing hospitalization, requiring a respirator or dying were calculated. All depending on the severity of their pathologies, just as it happens in the real world.

Experiments using this model can project the effects of different vaccines. And also what could happen if during the duration of the pandemic only a fraction of the population received it.

The results show to what extent vaccines with different levels of efficacy would have an impact on the pandemic. Furthermore, these conclusions could also be used to calculate the impact of the vaccine on aspects such as the number of people who would become infected and the costs and results of health policies. In this case, we assume that only one vaccine would be necessary.

What will it take to stop the pandemic?

Under normal conditions, both in epidemic and pandemic situations, and the more people are exposed to the virus, the number of daily new infected increases steadily until it reaches a peak and begins to decrease. But, of course, the time this process can take depends on how the virus itself evolves as well as the ways to combat it.

To stop the pandemic it is necessary that the number of new infected people drop to zero (or at least to a very low number) as soon as possible.

If the COVID-19 pandemic was starting and the percentage of infected population was close to 0%, simulations show that the efficacy of the vaccine should be at least 60% to stop the coronavirus, and the entire population should be vaccinated. .

This scenario is probably not possible given that, in the first place, there is a percentage of people who could not be vaccinated due to age or health problems. And second, many people would refuse to be vaccinated.

If only 75% of the population were vaccinated, the efficacy of the vaccine would have to be around 70%. And if only the 60% vaccine were delivered, the efficacy threshold would have to be even higher, at around 80%. The key is to make sure that the chain of contagion of the virus is stopped.

These figures are based on the premise that each infected person infects an average of 2.5 more people. Thus, if the virus became more contagious, the vaccine would in turn have to be more efficient.

However, the longer the pandemic lasts, the less room there will be to reduce its incidence with respect to its maximum level. It is like when you climb a mountain, which already parts of a certain height. Also, it is more difficult to end a pandemic when there is an increasing number of people who can spread it.

Thus, when around 5% of the population is already infected with the virus, the best thing to do is to reduce the incidence of the virus by around 85% compared to its peak level. The difference between 0% and 5% of infected involves millions of infections. Until now there is evidence that around 1% of the United States population is infected, although there are government sources that believe the actual percentage is much higher.

The percentage of people who get vaccinated will be key

With all of the above in mind, a vaccine as low as 60% effective could serve to curb the pandemic and allow society to return to normal. However, for this to be the case, most, if not all, of the world’s population would have to be vaccinated.

This seems unlikely considering that surveys indicating that only three quarters of Americans claim that they would agree to receive a coronavirus vaccine if they were assured that it was safe.

With fewer people protected, the vaccine would have to be at least 80% effective in order to stop the pandemic on its own, that is, completely eliminating social distancing measures. This information could serve as a reference to set a target for the development of the different coronavirus vaccines.

Again, this does not mean that a vaccine with a lower efficacy should be considered useless. It would only mean that social distancing measures and the obligation to wear a mask would continue until the pandemic naturally subsided or until a vaccine appeared. good enough.


This article was originally published in The Conversation. You can read the original here.

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