Collective immunity: where are we?

At the start of the pandemic, we heard a lot about collective immunity. Once a large part of the population is vaccinated or infected, we will be rid of the virus. According to a September 2020 post from the Pasteur Institute, a benchmark in infectious disease, it would have been enough for 67% of the French population to be vaccinated for the virus to stop circulating. Today, 90% of the adult population is vaccinated in France and … the government plans to introduce a New Year’s Eve curfew to prevent clusters. Explanations.

The concept of collective immunity

According to the Pasteur Institute, an infectious agent stops circulating when ” the fraction immune to this agent is greater than or equal to: 1-1 / R0 “. The R0 corresponds to the number of reproduction, that is to say the number of non-immune people that an infected person can infect, in the absence of barrier gestures. Just before the first confinement in February 2020, the R0 of the original virus was 3. This is how the Pasteur Institute had calculated this exciting rate of 67% of people immune to stop the virus.

In the meantime, different variants have appeared with ever higher R0s. The Alpha variant discovered in Great Britain has an R0 of 4.5; the Delta variant discovered in India has an R0 between 6 and 7 depending on the sources. With the Delta variant, it was already necessary that 85% of the population be immunized for the virus to stop circulating.

The R0 of Omicron variant has not yet been defined but it could well be above 10, according to a publication from December 17, 2021 in the prestigious The Lancet. This therefore means that 90% of the population must be immune to the virus to begin to hope for a return to normal.

90% of the population immune … against the variant in circulation!

90% of the population immune? That’s good, this is almost the case in France! Unfortunately, things are more complex. Collective immunity would work if 90% of the population were immune … to the Omicron variant. The latest data show that the effectiveness of vaccines is zero after two doses ofAstraZeneca, by only 35% after two doses of Pfizer-BioNTech, and 70% after 3 doses of Pfizer-BioNTech.

In addition, people who have already contracted Covid-19 with another variant could be infected a second time with the Omicron variant. Only people who have received their booster dose benefit from relative protection, ie 34.4% of adults on December 21, 2021. We are therefore far from 90%.

The fact remains that vaccination remains an essential tool in the fight against the pandemic. Even imperfectly protected, people vaccinated transmit the virus less and do less serious forms. In addition, limiting the circulation of the virus helps to delay theemergence new variants, necessarily more contagious and potentially more virulent.