How effective must the corona vaccine be to stop the pandemic? – Science

The containment of the corona pandemic depends on several factors.

We are looking forward to a corona vaccine to get the virus under control. However, the development of an effective and safe vaccine is in itself insufficient. Enough people also need to be vaccinated. How many people will depend on the effectiveness of the vaccine, experts say. The more effective such a vaccine, the faster we can achieve herd immunity.

According to new US research, the corona vaccine must be at least 70% effective and then vaccinated 80% of the population with it to stop the corona crisis, at least without regard to other control measures. These include the waiver rule, face masks and quarantine (potentially) infected people.

How should we interpret this news?

You get a vaccine primarily to protect yourself. The more effective the vaccine, the fewer vaccinated people will become ill after they become infected. The measles vaccine, for example, is a very effective vaccine (between 95 and 100%): out of 100 people who are vaccinated and come into contact with the measles virus, less than 5 will get measles.

When enough people are vaccinated, the effect of group immunity also occurs: the virus cannot spread among the vaccinated population. Moreover, if that group is large enough, then the vaccinated persons indirectly also protect the non-vaccinated population.

After all, such a virus needs susceptible people to survive. These are increasingly difficult to find among a well-vaccinated population. Unvaccinated people are protected thanks to those around them who have been vaccinated or who have gone through the disease naturally.

If a vaccine is less effective, a larger number of vaccinated persons will still be susceptible, and those people will not contribute to herd immunity. They can still get sick, and the virus can survive and spread further. More people must therefore be vaccinated to achieve herd immunity.

In order to slow down the corona crisis with a vaccine, and to build up group immunity of 60-70%, we therefore need the most effective vaccine and enough people who can be vaccinated. The more effective such a vaccine, the faster the critical threshold is reached for obtaining herd immunity. This has important advantages:

• We are getting closer to the number of vaccines.

• We can better prioritize.

• The number of vaccine refusers is less likely to compromise herd immunity.

Using simulation models, the American researchers calculated how many people in theory should be vaccinated in function of the effectiveness of a corona vaccine:

• If the entire population could be vaccinated, the corona vaccine must have an effectiveness of at least 60% to stop the epidemic.

• If 75% of the population is vaccinated, the vaccine must be at least 80% effective. Moreover, that 75% must be well distributed over the entire population.

These calculations do not take into account other corona measures, which of course also have their effect. It is also estimated that 10 to 15% of people in Europe have a natural background immunity because they have been through the infection. The researchers did not take those figures into account either.