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Americans are waiting in masks for the subway in New York, where the infection with the delta variant is growing threateningly. PHOTO: Reuters
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When faced with the highly contagious Delta variant, it seems illusory to achieve collective immunity through vaccines alone, but they are still crucial to curbing the COVID-19 pandemic, experts say.
For months, collective immunity, in other words the number of people immunized, after which the epidemic ceased to spread, has been considered the holy grail, a way out of the crisis. Like the Grail, however, isn’t this just a chimera? It all depends on the definition we adopt, experts say.
“If the question is, ‘Will vaccination alone reverse the course and control the epidemic?'” The answer is no, epidemiologist Mircea Sofona told AFP.
In fact, “two parameters are essential: the degree of infection of the virus and the effectiveness of the vaccine in terms of infection. However, this is not enough,” he added.
The already dominant variant Delta is estimated to be 60 percent more contagious than the previous one (Alpha) and twice as contagious as the original strain. By the way, the more contagious the virus, the higher the threshold required for collective immunity (which is obtained through vaccines or naturally – through illness).
“On a theoretical level, this is a very easy formula,” epidemiologist Antoine Flao told AFP.
The calculations are made on the basis of the virus’s reproductive rate (or R0), which is the number of people an infected person infects in the absence of control measures.
Decreased efficiency
For the original virus (at R0 of 3), the collective immunity threshold is estimated to be “66 percent” immunized, says Professor Flao. But “if R0 is 8, as in the Delta variant, we get to 90 percent,” he added.
This threshold can be reached if vaccines are 100 percent effective against infection. But this is not so.
According to data released Tuesday by US authorities, the effectiveness of Pfizer and Modern vaccines against coronavirus has dropped from 91 percent to 66 percent since Delta became the dominant option in the United States.
In addition to the features of the option, this may be due to the fact that efficiency decreases over time: it drops from 88 percent to 74 percent after five to six months for Pfizer and from 77 percent to 67 percent after four to five months for AstraZeneca. , is clear from a British study published on Wednesday.
This is forcing more and more countries to consider taking a booster (usually a third) dose.
All of these parameters lead to mathematical absurdity: to achieve collective immunity without any anti-vivid measures, “it is necessary to vaccinate more than 100 percent of the population,” Sofona said.
This is a goal that is illusory and according to one of the fathers of the AstraZeneca vaccine.
“With this option, we are in a situation where collective immunity is not possible because the strain infects vaccinated people,” Professor Andrew Pollard of Oxford University told British lawmakers on August 10.
“Myth”
However, even if herd immunity through vaccination turns out to be a “myth,” as Professor Pollard says, experts say vaccines are key.
“Scientists recommend that as many people as possible be immunized,” says Professor Flao.
First, vaccines remain very effective in preventing severe forms of the disease and hospitalizations.
Second, they provide collective protection for those who cannot benefit from vaccination: this applies to people whose immune systems are weakened by another disease (cancer or transplantation, for example).
Finally, it remains possible “to achieve collective immunity, but not only through vaccination,” Mircea Sofona said.
This implies maintaining the requirement to wear “masks and rules for social distance, especially in certain areas”, in order to control the virus and thus minimize the risks.
During the AIDS pandemic, when scientists said condoms had to be put on, many people replied, “Okay, for now, for a while,” but eventually continued to do so, Antoine Flao said, adding: “It’s possible to continue wearing masks indoors and in transport for a long time “.
/ BTA /
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