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.
The best known estimates, initially around 60% to 70%, have gone up over time. US officials have recently avoided citing a specific number, emphasizing the need to get the vaccine as soon as possible. “I would like people to get rid of this idea of referring to something that, by its very definition, is quite skittish,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, at a press conference at the White House on 12 Of april. A return to normality will be gradual, he says.
There is no doubt that widespread vaccination will reduce the pandemic, strengthen the economy and allow for more normal social activities. The number of deaths and serious cases has plummeted in Israel, which is leading the world race to immunize its population.
However, in the largest and most diverse USA, vaccines are becoming a polarized theme. Although more than a quarter of the population is fully immunized, the reluctance to take the vaccine can halt more complete adoption. (The groups most likely to say they will abstain from taking it are white Republicans and evangelical Christians, according to surveys). The vaccine supply already exceeds the demand in some regions of the country.
“As we move forward, we will have pockets where people will not have been vaccinated, divided by race, income and religion, unfortunately,” says Ali Mokdad, an epidemiologist at the Seattle Metrics and Health Assessment Institute. “We will automatically see outbreaks between these groups, or much higher infections between them, and we will never achieve herd immunity.”
Globally, the vaccination campaign has just begun, with enough doses applied to cover just 6% of the world’s population, according to Bloomberg’s vaccine tracker. These doses are concentrated in a handful of rich countries. “We were caught with pants down,” says Saad Omer, director of the Yale Institute for Global Health, who notes that very little has been spent on worldwide vaccine distribution, taking into account that viruses do not pay attention to borders. “The idea that we can have a stronghold of immunity in America gives me a knot in the brain and learn to live with the risk of imported variants.”
Recent vaccine setbacks do not help. Many countries have been limiting the use of the vaccine against adenovirus from AstraZeneca after it has been linked to rare blood clots. The American distribution of the Johnson & Johnson vaccine has been paused while authorities investigate a series of similar events associated with that vaccine. Doctors who advise the Centers for Disease Control and Prevention plan to meet on April 23.
In the long run, the biggest wildcard are the variants, especially those that decrease the vaccine’s effectiveness. After the exciting results of vaccine testing at the end of last year, “I thought we would practically see a return to normal lifestyle in the second half of 2021,” says virologist David Ho, who heads the Aaron Diamond AIDS Research Center. at Columbia University, where he has been studying variants. While Ho still expects continuous improvement, he says the explosion of variants has “diminished my optimism” from a swift conclusion to the pandemic.
The rainforest city of Manaus, Brazil, shows how difficult it will be to end the virus without high vaccination rates. The city was hit hard by the covid early in the pandemic, and a study concluded that 76% of the municipality was infected as early as October. This should have made the city an area for herd immunization. However, in December a new wave appeared, driven by the P.1 variant, which suggests that there are people who have caught the disease twice.
“It is not possible to understand Manaus without considering some cases of reinfection”, says Ester Sabino, a researcher of contagious diseases at the University of São Paulo. What is imagined is that vaccines offer more protection than natural immunity.
In an optimistic scenario, the coronavirus may end up with no way to continue mutating. However, it is likely that the complicated picture in the medium term will mean a turbulent and zigzagging path towards normality. It will involve easing restrictions in some places while brainstorming new ways of living in a world where the virus is still at large.
Over time, the coronavirus can become something similar to influenza, changing constantly so that an endless series of preventive injections is needed. Or, who knows, SARS-CoV-2 can settle down and turn into another common flu.
With so many possible results, researchers say the best strategy is to focus on what is closest to our control: increasing the number of vaccinations in the population. “From an immunological perspective, it is very simple: Vaccinate as many people as possible,” says Shane Crotty, a professor at the La Jolla Institute for Immunology. “Everyone will be better off if we manage to vaccinate 90% of the population.”
Translation by Fabrício Calado Moreira
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