The “utopian” dream of collective immunity to COVID-19 | Coronavirus

At first glance, the idea seemed promising. Since SARS-CoV-2 is so prevalent in the population, why not let the vast majority become infected with COVID-19 to build herd immunity and thus hasten the end of the pandemic?

This is the approach Sweden decided to take early in the pandemic, with results that have drawn criticism from many scientists. (New window) because of the high death rate resulting from this choice. With almost 19,000 dead, Sweden has a death rate per 100,000 inhabitants that is almost four times higher than Norway, where health measures were stricter.

Until recently, this did not stop many countries from seeing the exponential number of infections caused by the Omicron variant as a sign that the world had taken a step towards herd immunity.

However, according to Alain Lamarre, professor-researcher specializing in immune responses and virology at the National Institute of Scientific Research (INRS), collective immunity is almost impossible to achieve at this stage of the pandemic and would not allow the virus to spread anyway. to eliminate.

More and more people believe that herd immunity in variants is a utopia, says Lamarre. I think it will be hard to achieve it as people hear it, [c’est-à-dire] to reach a stage where the virus is no longer transmitted because non-immune people are indirectly protected by those who have been immunized.

Last March, Dr. Luc Boileau, acting director of public health for Quebec, also in an interview with: All in one morning that herd immunity could not be stably achieved.

Multiplication of mutations and reinfections

In fact, there are several reasons why herd immunity is not possible with SARS-CoV-2.

First, too many people believed we could achieve herd immunity like we did with measles, said Catherine Hankins, a professor of public health and public health at McGill University and co-chair of the group work on immunity to COVID-19. The reality is very different with SARS-CoV-2.

Since the start of the pandemic, remember, a new variety of interest or concern has appeared every four to six months. The virus has therefore not yet mutated and scientists are currently monitoring several Omicron subvariants and recombinants.

This virus keeps changing. However, the only viruses for which we have managed to achieve collective immunity are relatively stable viruses.says Mrs. Hankins before giving the example of measles or smallpox. In the case of these viruses, infection provided lifelong immunity.

In an ideal world, SARS-CoV-2 would have mutated few and only once infected people.

However, it is now clear that reinfections are part of the equation, as the immunity conferred by SARS-CoV-2 infection is usually short-lived. Although you have some protection against infection by the other tribes, you are not protected against Omicronsays Hankins.

She adds that this immunity actually relies on the next variant. Gold, we don’t know the properties of these new variantsshe says.

With the rate at which SARS-CoV-2 mutates, achieving herd immunity by relying on infections therefore becomes an endless cat-and-mouse game. Indeed, every time a new variant appears, everything has to be started again.

Too contagious for such a strategy

The highly contagious nature of the virus compared to others that humanity has faced also plays a role.

The higher the reproduction rate — the average number of people an infected person transmits the virus to — the greater the proportion of the population that needs long-term protection to achieve herd immunity, says Jane Heffernan, a disease modeling expert from the University of York studying the immune response to the COVID-19 vaccine using mathematical models.

However, SARS-CoV-2 has reached very high transmission rates, putting the threshold for herd immunity out of reach, she says.

The original strain of COVID-19 had a reproduction number between 2.0 and 2.5, then it doubled to about 5 for Delta (New window)† With the BA1 (New window)it is estimated that this percentage is 9.5, and with the BA.2 between 16 and 20.

In comparison, the flu has a rate between 2 and 3, and chickenpox between 10 and 12.

Measles, considered one of the most communicable diseases, has a reproduction rate of about 15. It is estimated that a vaccination coverage of more than 90% is needed to prevent measles outbreaks.

According to Ms. Heffernan, therefore, nearly 100% of the population would need to have long-term immunity to meet the current definition of herd immunity. Impossible, she said.

Also, Lamarre says, it’s difficult to know for sure what proportion of the population has been immunized after an infection because there isn’t enough testing and many people are asymptomatic. In addition, immunity declines rapidly after infection and varies from person to person and species to species. So how do you determine if the percentage of people who are immune is enough?

Vaccines and Transmission

At the start of the vaccination campaign, it was hoped that the pandemic would be under control if 75% of the population were vaccinated.

While vaccines protect well against severe symptoms of the disease, they cannot prevent all transmission, especially with Omicron, which is highly contagious.

dr. Luc Boileau specifies that 10% to 15% of adequately vaccinated people can contract the virus and thus transmit it.

The concept [d’immunité collective] only works if you are protected against transmission. This is not the case.

A quote from Alain Lamarre, INRS

In addition, if the immunity of a vaccine lasts longer than that obtained after an infection, it is not lifelong, as is the case with measles, for example.

To cope with future surprises from SARS-CoV-2, effective vaccines must be designed against a large number of variants, Heffernan says.

Mr Lamarre agrees. We are not going to eradicate the virus: it is not a realistic goal. On the other hand, if we develop a vaccine that better controls transmission, it will be possible to slow the spread and reduce the peaks of infections.

A strategy discredited by studies

While all signs point to herd immunity not being achieved in the case of COVID-19, this idea has cropped up in public debates several times during the pandemic. She has also often been discredited.

In October 2020, the Great Barrington Declaration advocated for the majority of the population to become infected while protecting the most vulnerable. This proposal has been strongly criticized, mainly on moral grounds. Is it ethical to sideline the most vulnerable so the rest of the people can do their business?, the opponents asked? (New window)

A recent analysis published in the journal Nature (New window) noted that Sweden has failed to adopt this strategy. The authors claim that uncontrolled transmission has caused many preventable deaths.

In May 2020, a senior World Health Organization (WHO) official also recalled that the definition of herd immunity should not be applied to natural infections. Herd immunity achieved through infections does not put people, life and suffering at the center of this equationsaid Mike Ryan, WHO emergency director.

Even before the advent of vaccines, several scientists had claimed that the lancet (New window) Which any pandemic management strategy that relies on immunity conferred by natural infections is flawed

Trying to achieve herd immunity through infection is a dangerous mistake unsupported by scientific evidence.

A quote from A study published in The Lancet

In another study published this week (New window)Researchers in the UK have modeled the impact of a strategy that allowed high infection rates among the lower-risk population in hopes of protecting the vulnerable. We therefore looked into whether it was appropriate to targeted protection to reduce health measures in the general population.

The authors are adamant that this approach is heavily biased and would have been devastating to the elderly and those with medical histories.

In fact, the UK is said to have seen more than 50,000 additional deaths, the vast majority of vulnerable people. This is likely a conservative estimate, the authors say, as hospitals and intensive care units would be quickly overwhelmed. [par une hausse importante du nombre de cas]†

They explain that this strategy has three flaws. First, it is impossible to perfectly protect all the most vulnerable people. For example, residents of long-term care facilities are necessarily in contact with staff, and many high-risk people live in the community with low-risk people.

The second weakness is that in the event of a major resurgence in the number of cases, many people would most likely have changed their behavior to avoid infection. This would have caused smaller and longer waves, with less overall infections. Thus, the levels necessary to obtain herd immunity would never have been reached.

The third weakness mentioned is that herd immunity only provides indirect protection and is only temporary. Once protective measures for vulnerable people had been lifted, they would again have been at risk of being infected by residual transmission in the community. For example, residents of long-term care facilities would have remained at risk indefinitely.

For all these reasons, these three experts say governments cannot rely on previous infections to prevent new waves. Vaccination remains the best tool, they say, and vaccines will need to be improved until the virus becomes endemic.

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