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The coronavirus vaccine is eagerly awaited by the entire planet. The return to normalcy in our daily life will depend on him. But should the entire population be vaccinated to protect us or just a part? Its effectiveness will depend on several criteria.
The 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 systems to make proteins, antibodies, which fight diseases.
The percentage of people who need to have antibodies to achieve herd immunity depends on each disease. “For example, herd immunity to measles is achieved when approximately 95% of a population is vaccinated. The remaining 5% is protected because measles does not spread among the vaccinated,” explains CHI.
For Covid-19, we lack hindsight to give a precise figure. Collective immunity depends on the basic reproductive rate of the disease, the famous R0. It is the average number of people a patient will infect. The higher this R0, the higher the percentage of people immunized with a vaccine should be.
It all depends on the effectiveness of the vaccine
Covid-19’s R0 is 3.3 (it is 2 for seasonal flu 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, an epidemiologist at the School of Advanced Studies in Public Health (EHESP). If the R0 drops to 1.5 when the people wear masks and respect the limits set in place, so only 33% of the population will have to be vaccinated, ”he explains.
But this percentage can still vary depending on the effectiveness of the vaccine. If a vaccine is only 70% effective, more people will need to be vaccinated than if it were 90 or 95% to gain collective immunity.
Another imperative, and not least, the vaccine must protect well from the transmission of the virus and not only from the serious effects of Covid-19. “There are clinical trials of vaccines on monkeys that have made it possible to eliminate the coronavirus from the lungs but not from the respiratory tract. In this case the virus would continue to spread,” emphasizes Pascal Crépey.
An effective vaccine will therefore be a serious asset in reducing the coronavirus pressure on the world population, but will not initially guarantee full herd immunity. It will be necessary to continue to take 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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