With the delta variant, much more contagious than the original strain, it seems illusory to achieve collective immunity with only anticovid-19 vaccines, although immunizing agents continue to be crucial to contain the pandemic – highlight specialists heard by the AFP.
For several months, collective immunity – that is, the level of immunized people at which the epidemic is controlled – has been considered the “holy grail” for a way out of the global health crisis. But, like the grail, isn’t it a chimera? It all depends on the definition adopted, the scientists reply.
“If the question is ‘will only the vaccines allow the setback and control of the epidemic?’, the answer is no,” epidemiologist Mircea Sofonea told AFP.
In fact, “there are two parameters: the intrinsic contagiousness of the virus and the effectiveness of the vaccine against the infection. And they are not enough”, he adds.
Why? The delta variant, now dominant, is considered 60% more transmissible than its predecessor (alpha), and twice as much as the original strain. And the more contagious a virus is, the higher the level needed to achieve collective immunity, obtained through vaccines, or natural infection.
“On a theoretical level, it’s a very easy formula to calculate,” says epidemiologist Antoine Flahault.
The calculation is based on the virus’s baseline reproduction index (or R0), that is, the number of people that an infected person infects in the absence of control measures.
– Reduction in effectiveness -For the original or historical virus (with an R0 of 3), the collective immunity mark was calculated at “66%” of immunized people, recalls Professor Flahault. “But if the R0 is 8, as with the delta variant, we get to 90%,” he explains.
This level could be reached if vaccines were 100% effective against the infection. But it’s not the case.
According to data published on Tuesday (24) by US authorities, the effectiveness of Pfizer and Moderna vaccines against covid-19 has dropped from 91% to 66% since the delta variant became dominant in the country.
In addition to the characteristics of the variant, the loss of effectiveness can also be caused by a reduction over time. Falls from 88% to 74% after five to six months for Pfizer; and from 77% to 67%, after four or five months, for AstraZeneca, according to a British study published on Wednesday (25).
This framework has been encouraging more and more countries to contemplate booster vaccines, in general, a third dose.
One of the fathers of the AstraZeneca vaccine, Professor Andrew Pollard of Oxford University, was clear in an Aug. 10 speech to British MPs: “With the current variant, we are in a situation where mass immunity is not a possibility , as it infects vaccinated people”.
– “Myth” -Still, even though collective immunity through vaccination has become a “myth”, in the words of Professor Pollard, experts insist that vaccines are indispensable.
“What scientists stand for is that we should have as many people protected,” said Professor Flahault.
Vaccines are very effective in preventing severe forms of the disease, as well as hospitalizations.
In addition, they guarantee collective protection for those who cannot benefit from vaccination, such as people with weakened immune systems due to another disease, such as cancer, or in the case of transplants, for example.
In short, yes, it is possible “to achieve collective immunity, but not only through vaccination”, considers Mircea Sofonea.
This means maintaining “the use of masks and forms of social distancing, especially in certain territories”, to curb the virus and reduce risks as much as possible.
“During the AIDS pandemic, when scientists said it was necessary to use condoms, a lot of people said, ‘It’s okay for now, for a while,’ but we kept doing it,” recalls Antoine Flahault.
“We can continue to wear the mask indoors and in transport for quite some time,” he added.