Collective immunity against COVID-19 has never been and will never be achieved, according to some experts. The hope of ending the pandemic by having large numbers of people infected is therefore not a good strategy.
At first glance, the idea seems promising. Since SARS-CoV-2 is prevalent in the population, why not just let the majority be infected with COVID-19 to develop 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 garnered criticism from many scientists. (New window) due to the high mortality rate caused by this option. With nearly 19,000 dead, Sweden has a death rate per 100,000 population that is almost four times higher than Norway, where health measures are more stringent.
This did not stop many countries until recently from seeing the exponential number of infections caused by Omicron as a sign that the world has taken a step towards herd immunity.
However, according to Alain Lamarre, professor-researcher who specializes in immune response and virology at the National Institute for Scientific Research (INRS), collective immunity is almost impossible to achieve at this stage of the pandemic and still not, to eliminate the virus.
More and more people believe that with variants, herd immunity is utopian, Lamarre said. I think it’s going to be hard to reach the way people hear it, [c’est-à-dire] to reach a stage where the virus is no longer transmitted because people who are not immune are not directly protected by those vaccinated.
In March, Dr. also said. Luc Boileau, acting director of public health for Quebec, in an interview with All one morning that herd immunity cannot be achieved firmly.
Multiplication of mutations and reinfections
In fact, there are several reasons why herd immunity to SARS-CoV-2 is not possible.
First, so many people believe we can achieve herd immunity like we did with measles, says Catherine Hankins, professor of public and population health at McGill University and co-chair of the Group work on immunity at COVID-19. The reality is somewhat different with SARS-CoV-2.
Since the pandemic started, it should be noted, a new variant of interest or concern appeared every 4 to 6 months. The virus is therefore not yet finished to mutate and scientists are currently monitoring several sub-variants of Omicron, as well as recombinants.
This virus is constantly evolving. However, the only ones for which we have managed to achieve collective immunity are relatively stable viruses.said Mrs. Hankins, giving the example of measles or smallpox. In the case of these viruses, the infection provided lifelong immunity.
In an ideal world, SARS-CoV-2 could mutate a little and only once infect people.
It is now clear, however, that reinfections are part of the equation, since the immunity conferred by SARS-CoV-2 infection is usually short-lived. Although you have some protection against infection from other strains, you are not protected against Omicronsaid Hankins.
He added that immunity to this disease really depends on the next variant. gold, we do not know the characteristics of these new variantshe says.
With the speed of SARS-CoV-2 mutation, achieving herd immunity by relying on infections therefore becomes an endless game of cat and rat. In fact, whenever a new variant appears, it has to start all over again.
Too contagious for such an approach
The highly contagious nature of the virus compared to others that humanity has had to deal with also enters.
The higher the reproductive rate – the average number of people infected with the virus by an infected individual – the higher the proportion of the population with long -term protection needed to achieve herd immunity, says Jane Heffernan, a modeling expert. of disease at York University studying. the immune response of the COVID-19 vaccine using mathematical models.
However, SARS-CoV-2 has reached very high transmission rates, pushing the herd immunity threshold not to be reached, he said.
The original strain of COVID-19 had a reproduction rate between 2.0 and 2.5, then it doubled to approximately 5 for Delta (New window). Using BA1 (New window)it is estimated that this rate is 9.5, and includes BA.2, between 16 and 20.
In comparison, the flu has a rate between 2 and 3, and chicken pox between 10 and 12.
Measles, considered one of the most contagious diseases, has a multiplication rate of approximately 15. It is estimated that more than 90% need vaccination to prevent the spread of measles.
According to Ms. Heffernan, therefore, nearly 100% of the population needs to have long -term immunity to meet the current definition of herd immunity. Impossible, he said.
Also, Lamarre says, it’s hard to know what proportion of the population is vaccinated after infection, because not enough testing has been done, and many people have no symptoms. Additionally, immunity after infection rapidly weakens and varies with each person, and is extremely strained. So how do you know if the proportion of people who are immune is adequate?
Vaccines and delivery
With the start of the vaccination campaign, it is expected that the pandemic will be controlled if 75% of the population is vaccinated.
However, while vaccines do well to protect against the chronic symptoms of the disease, they cannot prevent all transmission, especially to Omicron, which is highly contagious.
Dr. pointed out. Luc Boileau that 10% to 15% of people who are adequately vaccinated can get the virus and therefore transmit it.
” The concept [d’immunité collective] will only work if you are protected against transmission. This is not the case. “
Furthermore, if the immunity granted by a vaccine lasts longer than that developed after an infection, it is not lifelong, as is the case with measles, for example.
Dealing with future surprises from SARS-CoV-2 requires designing effective vaccines against multiple variants, Heffernan said.
Mr. Lamarre agrees. We will not remove the virus; that is not a realistic goal. On the other hand, if we make a vaccine that better controls transmission, it is possible to slow the spread and reduce the peaks of infections.
An approach that studies undermine
Although all indications are that herd immunity was not achievable in the case of COVID-19, this idea appeared in public debates on several occasions during the pandemic. He was also repeatedly insulted.
In October 2020, the Great Barrington Declaration advocated letting the majority of the population become infected, while protecting the most vulnerable. This proposal was strongly criticized, especially on moral grounds. Is it ethical to sideline the most vulnerable so other people can continue their business, opponents question (New window)?
A recent review published in the journal Nature (New window) noted the failure experienced by Sweden in using this technique. The authors say that unchecked transmission caused many deaths that could have been avoided.
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 is achieved through infections does not put people, life and suffering at the center of this equationsaid Mike Ryan, WHO emergency director.
Even before vaccines arrived, some scientists insisted The Lancet (New window) na 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 that is not supported by scientific evidence. “
In another study published this week (New window), UK researchers have modeled the impact of an approach where high infection rates are allowed in lower-risk populations, in the hope of protecting the vulnerable. We therefore examined whether it was appropriate to adopt a targeted protection to reduce health measures in the general population.
The authors argue that this approach is highly biased and may be detrimental for older people and those with a medical history.
In fact, the UK has seen more than 50,000 additional deaths, for the most part vulnerable people. This is likely a conservative estimate, the authors say, as hospitals and intensive care units are rapidly overwhelmed. [par une hausse importante des cas].
They explained that this technique had three flaws. First, it is impossible to fully protect all the most vulnerable people. For example, residents of long-term care facilities are required to communicate with staff, and many high-risk people live in communities with low-risk people.
The second weakness is that in the event of a large resurgence of cases, it is likely that many people have changed their behaviors to prevent infection. This resulted in smaller and longer waves, with fewer total infections. Thus, the levels needed to achieve herd immunity will never be reached.
The third weakness mentioned is that herd immunity provides only indirect protection and is only temporary. Once protective measures for vulnerable people are removed, they will again be put at risk of being contaminated by residual transmission to the community. For example, residents of long -term care facilities remain at risk indefinitely.
For all these reasons, these three experts said governments cannot rely on past infections to prevent new waves. Vaccination remains the best tool, they say, and vaccines need to be improved until the virus becomes endemic.
Source: Radio-Canada
