At first glance, the idea seemed promising. Since SARS-CoV-2 is so prevalent in the population, why not just let the vast majority become infected with COVID-19 in order to build herd immunity and thus hasten the end of the pandemic?
This is the approach that Sweden decided to take early in the pandemic, with results that have earned it criticism from many scientists. (New window) because of the high death rate caused by this choice. With nearly 19,000 dead, Sweden has a death rate per 100,000 population almost four times higher than Norway, where health measures were stricter.
That hasn’t stopped 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 specializing in immune responses and virology at the National Institute for Scientific Research (INRS), collective immunity is practically impossible to achieve at this stage of the pandemic and would not anyway, to eliminate the virus.
More and more people believe that with variants, herd immunity is utopian, says Lamarre. I think it’s gonna be hard to reach it the way 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 are immunized.
In March, Dr. Luc Boileau, acting director of public health for Quebec, also stated in an interview with All one morning that herd immunity could not be achieved stably.
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 that we could 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 to COVID-19. The reality is quite different with SARS-CoV-2.
Since the start of the pandemic, it should be remembered, a new variant of interest or concern has appeared every 4 to 6 months. The virus has therefore not finished mutating and scientists are currently keeping an eye on several sub-variants of Omicron, as well as recombinants.
This virus keeps changing. However, the only ones for which we have managed to achieve collective immunity are relatively stable viruses.
says Mrs. Hankins, 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 little and only infected people once.
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 the other strains, you are not protected against Omicron
says Hankins.
She adds that this immunity is actually dependent on the next variant. Gold, we do not know the properties of these new variants
she says.
At the rate at which SARS-CoV-2 mutates, achieving herd immunity by relying on infections therefore becomes an endless game of cat and mouse. Indeed, each time a new variant appears, everything has to be started over.
Too contagious for such a strategy
The highly contagious nature of the virus compared to others that humanity has had to face also comes into play.
The higher the reproductive rate — the average number of people an infected individual transmits the virus to — the higher the proportion of the population with long-lasting protection is needed to achieve herd immunity, says Jane Heffernan, a York University disease modeling expert who studies 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 out of reach, she says.
The original strain of COVID-19 had a reproduction rate between 2.0 and 2.5, then it doubled to around 5 for Delta (New window). With the BA1 (New window)it is estimated that this rate is 9.5, and with the 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, which is considered one of the most transmissible diseases, has a reproduction rate of about 15. It is estimated that a vaccination rate of more than 90% is needed to prevent outbreaks of measles.
According to Ms. Heffernan, therefore, almost 100% of the population would have to have long-lasting immunity to satisfy the current definition of herd immunity. Impossible, she said.
Also, says Lamarre, it is difficult to know for sure what proportion of the population has been immunized after an infection, because not enough testing is done, and many people are asymptomatic. Additionally, post-infection immunity wanes rapidly and varies from person to person, and strain to strain. So how do you determine whether the proportion of people who are immune is sufficient?
Vaccines and transmission
At the start of the vaccination campaign, it was hoped that the pandemic would be controlled if 75% of the population were vaccinated.
However, 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 therefore transmit it.
” The concept [d’immunité collective] only works if you are protected against transmission. This is not the case. »
Moreover, if the immunity conferred 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 a multitude of variants, says Heffernan.
Mr. Lamarre agrees. We are not going to eradicate the virus; that’s not a realistic goal. On the other hand, if we manufacture 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
Even though all signs indicate that herd immunity will not be achieved in the case of COVID-19, this idea has surfaced in public debates on several occasions during the pandemic. She has also been repeatedly discredited.
In October 2020, the Great Barrington Declaration advocated letting the majority of the population become infected, while protecting the most vulnerable. This proposal has been strongly criticized, in particular for moral reasons. Is it ethical to sideline the most vulnerable so the rest of the people can go about their business, opponents asked (New window)?
A recent analysis published in the journal Nature (New window) noted the failure experienced by Sweden in adopting this strategy. The authors claim that unchecked transmission caused many deaths that could have been avoided.
In May 2020, a senior official of the World Health Organization (WHO) 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 equation
said Mike Ryan, WHO emergency director.
Even before the arrival of vaccines, several scientists had asserted in The Lancet (New window) that 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 not supported by scientific evidence. »
In another study published this week (New window), researchers in the UK modeled the impact of a strategy in which high rates of infection would have been allowed among the population less at risk, in the hope of protecting the vulnerable. We therefore checked whether it was appropriate to adopt a 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 for older people and those with medical histories.
In fact, the UK would have seen more than 50,000 additional deaths, for the majority of vulnerable people. This is likely a conservative estimate, the authors say, as hospitals and intensive care units would have been quickly overwhelmed. [par une hausse importante des 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 necessarily have 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 large resurgence of cases, many people would most likely have changed their behaviors to avoid infection. This would have caused smaller and longer waves, with fewer total infections. Thus, the levels needed to achieve herd immunity would never have been reached.
The third weakness mentioned is that herd immunity only confers indirect protection and is only temporary. As soon as protective measures for vulnerable people were removed, they would again be 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.
Reference-ici.radio-canada.ca