New variants of coronavirus may delay the dream of collective immunity

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Vacinação no Museu de História Natural, em Nova York: a oferta de vacinas já supera a demanda em algumas regiões dos Estados Unidos. Republicanos e evangélicos são os grupos com maior relutância em tomar a vacina

Vaccination at the Natural History Museum in New York: the supply of vaccines already exceeds the demand in some regions of the United States. Republicans and evangelicals are the groups most reluctant to get the vaccine. (Gabby Jones / Bloomberg / Getty Images)

Long before herd immunity became humanity’s collective obsession, the phrase was already used to refer to sick cows. More than a century ago, veterinarians observed that outbreaks of a highly contagious bacterial infection that threatened livestock were stopped as soon as they burned a certain percentage of the herd, as long as new animals were not introduced. The concept was extended to a series of human outbreaks, and soon the term became a commonplace in epidemiology.

Since the beginning of the pandemic, the exact moment when USA must achieve herd immunity for the covid-19 has been discussed furiously in hearings in Congress, on TV shows and among various backyard epidemiologists on Twitter. In the popular imagination, the phrase has become a synonym for an end to the pandemic – a finish line that will suddenly make the virus recede and allow a return to normal without a mask.

However, considering how relentless, unpredictable and subject to mutations the coronavirus has been showing, leading researchers are beginning to say that a more realistic expectation for the final season of covid is a slow and gradual improvement, with several setbacks and setbacks. along the way.

Vaccines powerful companies, such as that of Modena and that of Pfizer’s partnership with BioNTech, are leaving the world in a much better position than it was six months ago. But it is likely that the virus – which has already killed 3.2 million people and infected more than 154 million worldwide, with no signs of slowing down – will continue to circulate for many years. In other words, the end of the pandemic may only become clear when you look back, not ahead.

The idea behind herd immunity is irresistibly simple. Once a certain percentage of the population becomes immune through vaccination or contagion – perhaps 70% to 85% of a population, in the case of this particular virus -, transmission becomes more difficult and the protective effect shields that community so wider.

The formula, 1-1 / R₀, where R₀ is the average number of new infections that are thought to be the result of each case, requires basic algebra. However, when looking at the details, in a short time this intuitive concept becomes complicated. “Everyone talks about herd immunity as if it were that really important threshold, but it’s actually a pretty rough number and hard to estimate,” says Nicholas Reich, a biostatistician at the University of Massachusetts in Amherst, who develops predictions of covid- 19 combining data from different research groups from around the world.

Humans are not cows. Far from being a simple fixed number, the percentage of the population needed to achieve herd immunity can vary over time and from place to place depending on a wide range of factors, including how long the immunity lasts, how people behave , what mitigating factors are in place, how quickly the virus mutates and even the local climate.

A look at the old vaccination campaigns offers a sober perspective on the job ahead. Smallpox is one of the largest human viruses that has been officially eradicated. Rare cases of polio still occur in some countries. Even measles took years to be fully contained in the United States thanks to potent vaccines.

Whatever the theoretical number may be, it has risen in recent months due to the rise of more infectious variants, such as the B.1.1.7 strain that predominates today in the USA. At the beginning of the pandemic, some optimistic studies – generally cited by opponents of the lockdowns – claimed that up to 10% or 20% of the infected population would already be sufficient to lead to herd immunity.