Herd immunity is still far away


In recent weeks, several countries have launched surveys on representative population samples to more accurately estimate how many have so far been infected with the coronavirus. According to the first analyzes, the percentage of infected people is quite low and far from the threshold beyond which it becomes more difficult for a contagious disease to spread (the so-called “flock immunity”). Investigations and studies of this type help to better understand how widespread COVID-19 is, considering that tests using swabs offer a very partial view on the actual trend of contagion in the various countries.

Herd immunity
Herd immunity indicates the ability of a group of people to resist a particular infection, against which a high percentage of the population is immune. Usually this result is obtained when a large part of the population contracts an infectious disease and heals, or is vaccinated obtaining immunity without having to get sick (avoiding the risks that follow). Immunity causes the transmission of the infectious agent to be reduced because most individuals have developed antibodies to stop it, before it can replicate and make them contagious, thus protecting even people who have not fallen ill or who are not have been able to vaccinate. With a sufficiently high coverage of a vaccine, for example, a virus can disappear until the complete eradication of the disease causing it.

Immunity and vaccines
According to numerous experts, 60-80 percent of the population must become immune to the virus to develop herd immunity from coronavirus. This circumstance could allow you to stop COVID-19, but there are two things to keep in mind.

The first is that we are still not sure that we can become immune to the current coronavirus: some indications provided by the healed patients leave researchers optimistic about the immunization process, but it will still take months to understand if and for how long our immune system maintains a remember the coronavirus and therefore know how to stop it before it can do damage, in case of a new infection. The second is that to date there are no effective vaccines against coronavirus: until we have one available, the possible immunity of flock can only be achieved through the normal spread of the disease.

Where are we at
Aware of these circumstances, some research groups have however worked on analyzes and statistical models to estimate the current percentage of the population with antibodies against coronavirus, in various parts of the world. As reported on New York Times, no major city surveyed comes close to the 60 percent threshold: in New York it is estimated that just under 20 percent of the inhabitants developed antibodies to coronavirus, in London 17.5 percent, in Madrid l ’11, 3 per cent and in Stockholm just over 7 per cent.

The city of New York has been one of the most affected by the pandemic with over 200 thousand cases detected, yet from the elaborations of the research it seems that only a fifth of the population has developed the antibodies. The data is based on an epidemiological survey conducted in the first days of May in selected areas of the city, but is however considered representative of the situation as a whole.

How much time
Many epidemiologists agree that protection given by flock immunity cannot be achieved quickly. In addition to being unclear if and however immune you are, the researchers have not yet established with certainty what the threshold may be: the most optimistic evaluations speak of 60 percent, but there are those who believe that the percentage could be highest and around 80 percent. However, there is no univocal data: depending on the characteristics of the population, the population density and other variables, each geographic area has its own threshold for achieving herd immunity. The 60 per cent figure is therefore an average figure.

The presence of antibodies against coronavirus is detected through a serological test: a normal blood sample is taken and the sample is then analyzed looking for traces of an immune response from the body. These tests let you know if you have ever come into contact with the coronavirus, even after several weeks from the possible infection, which could then have caused the disease (not everyone gets sick of COVID-19, or realizes they get sick because develop very mild symptoms). They are therefore different from tests using a swab, which instead serve to find out if you are infected when taking saliva and mucus.

There are different types of serological tests available and not all of them are reliable in the same way: this could therefore affect the first research conducted to evaluate the spread of the infection among the population. Epidemiological investigations, on a large scale and with statistical criteria, based on serological tests can however offer important information for the study of the pandemic and its effects in countries with different populations and characteristics.

In Italy, the Ministry of Health has launched its own epidemiological investigation, with the collaboration of ISTAT and selecting a sample of 150 thousand people throughout the national territory.

Immunity and influence
The presence of flock immunity does not imply that susceptible people stop running the risk of getting COVID-19 overnight. Each non-immune individual maintains the risk of getting sick, but still becomes less likely to be exposed to the coronavirus because fewer contagious people are around.

Even if 60 percent of people with coronavirus antibodies were not reached, a significant percentage of immune individuals would still help reduce the rate at which the disease spreads, avoiding the risk of new peaks in the health emergency with hospitals full of patients and in difficulty to take care of everyone.

Seasonal flu is a disease other than COVID-19 and is caused by a different type of virus, but it can be a useful example to better understand the effects of flock immunity. Every year new variants of the viruses that cause them occur, and this is one of the reasons why you get sick several times in the course of your life: mutations mean that the immune system must learn every time to fight against viruses. flu. This is why a different vaccine is produced every year, calibrated to be specific against the flu viruses that are expected to circulate more among the population in the cold season.

All these circumstances mean that immunity to flocks is never achieved against the flu that prevents influenza viruses from remaining in circulation. The possibility of vaccinating a significant portion of the population at risk and the ability to develop immunity after falling ill, of a specific variant, means that only a part of the population falls ill with flu every year. As a result, fewer people fall ill in absolute terms than would occur in the absence of a vaccine, resulting in fewer complications from the disease and ultimately fewer deaths.

New disease
However, COVID-19 is a completely new disease, for which we still have weapons that have sprouted: there is no vaccine, most of the population has not yet come into contact with the coronavirus that causes it and has not developed antibodies. This implies that a much larger portion of the population is at risk and will continue to be at risk for many months.

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