Coronavirus: what part of the population must be vaccinated to achieve collective immunity?

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The coronavirus vaccine is eagerly awaited by the whole planet. On him will depend the return to normal in our daily life. But should the entire population be vaccinated to protect us or just a part? Its effectiveness will depend on several criteria.

Collective immunity obtained thanks to a vaccine makes it possible to make a disease more rare and therefore to save lives. Vaccines train our immune system to make proteins, antibodies, that fight disease.

The percentage of people who must have antibodies to achieve herd immunity depends on each disease. “For example, herd immunity to measles is achieved when about 95% of a population is vaccinated. The remaining 5% is protected because measles will not spread among those vaccinated,” explains the WHO.

For the Covid-19, we lack hindsight to give a precise figure. Collective immunity depends on the basic reproduction rate of the disease, the famous R0. It is the average number of people that a patient will infect. The higher this R0, the higher the percentage of people immunized by a vaccine should be.

It all depends on the effectiveness of the vaccine

The R0 of Covid-19 is 3.3 (it is 2 for seasonal influenza and 15 to 20 for measles). “In this case, it would take about 60-70% of the population to be immunized with a vaccine, according to Pascal Crépey, epidemiologist at the School of Advanced Studies in Public Health (EHESP). If the R0 drops to 1.5 when people wear the mask and respect the limitations put in place, so only 33% of the population will need to be immunized with a vaccine, ”he explains.

But this percentage can still vary depending on the effectiveness of the vaccine. If a vaccine is only 70% effective, it will be necessary to vaccinate more people than if it is 90 or 95% to achieve collective immunity.

Another imperative, and not the least, the vaccine must protect well against the transmission of the virus and not only against the severe effects of Covid-19. “There are clinical trials of vaccines on monkeys which have made it possible to remove the coronavirus from the lungs but not from the respiratory tract. In this case, the virus would continue to spread,” said Pascal Crépey.

An effective vaccine will therefore be a serious asset in reducing the pressure of the coronavirus on the world population, but it will not initially guarantee total herd immunity. It will be necessary to continue to adopt barrier measures to prevent its spread and to continue to seek treatment against Covid-19 in parallel. The world after the coronavirus is not for tomorrow.

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