The question of group immunity has frequently returned since the epidemic began. It is this strategy to keep the virus circulating in the population until a majority of people are infected and have developed a natural immune response so that the virus stops spreading. even that was chosen by certain countries, particularly the UK, Sweden or the Netherlands. However, this strategy involves costs that are very difficult for human life, so Britain has just backed out and Sweden and the Netherlands have just taken stricter social distance measures.
Let us return to this concept of group immunity today. Since in the absence of a vaccine or rather while waiting for a vaccine until a majority of the population has not attained this natural immunity, the abolition of containment measures can at any time lead to a resumption of a contagion wave and thus new epidemic spikes.
What is group immunity?
To understand this well, we need to return to the three parameters that define the spread of a new epidemic, a newly emerging virus that nobody is immune to. The number of contacts, the risk of infection and the duration of the contagious period.
We know these parameters relatively well. Based on the start of the epidemic in Wuhan, the team of Neil Ferguson, epidemiologist at Imperial College London, does the following calculation: Assuming that the average SARS-CoV2 5 incubation time is 1 day, this risk of infection begins 12 hours before the first Symptoms in people who declare the disease and 4.6 days after infection in asymptomatic people with an average infection duration of 6.5 days of reproduction, which is referred to as R0, would be about 2.5.
This clearly means that a person infected with SARS-CoV2 transmits the disease on average between 2.4 and 2.6 other people. If we associate this with a population group, 1000 infected people with an R0 of 2.5 infect 2500 people, then 6250 and so on.
To stop the epidemic, this R0 must fall below 1. If a patient infects less than another person, the curve is reversed. With an R0 of 0.5, 1,000 people contaminate 500, then 250 and the epidemic ends.
As a hint, remember that the R0 value of influenza is 1.3, that of HIV between 2 and 5 and that of measles over 12.
We therefore understand that the more a large part of the population has acquired this immunity, the more the R0 decreases, since some of the contacts can no longer catch the disease. So if half of the population is immune. The SARS-CoV2 R0 would drop to 1.25, which is not enough to fall below 1. We can therefore estimate that with a simple calculation we can get an immunity rate to stop the epidemic group to 60% of the population.
What problems with the strategy of group immunity?
There are a number of problems here. The first is obviously that of mortality. Even if it is currently impossible to calculate the exact death rate of Covid-19, as I explained to you in an earlier column, it would lead to free circulation of the virus to reach hundreds of thousands of deaths, 60% of the infected population, a number , which obviously multiplies with the overloading of the emergency services, because given the high risk of infection of the virus, everyone would be infected very quickly and in a relatively short time.
The spread of the virus must therefore be slowed down as much as possible to spread the occurrence of serious cases in intensive care units. This is exactly what the containment measures of most states are designed for. But then there is another problem: If the population is limited and this restriction inhibits the progression of the epidemic, what happens if the restriction ceases? A group immunity could not be determined because the virus is not widespread and there is a risk of an epidemic rebound and a new outbreak of contamination.
To avoid this scenario, the Neil Ferguson team suggests an intermittent containment solution: Prefer a series of small epidemics that are interrupted by new social distance measures to spread this group immunity over time until the production of a permanent vaccine.
But there are also other questions. In particular, the duration of the immunity acquired from SARS-CoV2. How long do you stay immune after getting the virus? A few weeks like a cold? A few months like the flu? Or all of life like measles? Scientists have no idea yet. It all depends on the ability of the virus to mutate – this is the case with flu and the reason why you need to be vaccinated every year. It is also currently unknown whether a person who has been infected with the virus is likely to be infected a second time. Several cases have been reported, particularly in China and Japan, but it is currently impossible to say whether partially cured people are a second infection or a resurgence of the virus.
The case remains China, which no longer has an official case of contamination without reaching the group immunity threshold. However, this result has been achieved through radical measures to fully contain, systematically detect, control and generally monitor infected populations and their contacts.
Then what solution?
For the time being and to the best of our current knowledge, we must delay the spread of the virus as much as possible and count on a progressive acquisition of collective immunity that spreads over time to manufacture and manufacture, the massive distribution of vaccines for the entire population to ultimately to achieve immunity to the vaccine group.
Nicolas Martin and the team from The Scientific Method