Herd Immunity Is Humanity’s Great Hope, and It’s Proving Elusive

Long before herd immunity became humanity’s shared obsession, the phrase referred to sick cows. More than a century ago, veterinarians observed that outbreaks of a highly contagious bacterial infection menacing cattle died down once they’d burned through a certain percentage of a herd, so long as new animals weren’t introduced. Soon the concept was extended to a variety of human outbreaks, where it became a staple of epidemiology.

Since the beginning of the pandemic, exactly when the U.S. might reach herd immunity for Covid-19 has been furiously debated in congressional hearings, on TV shows, and among the many armchair epidemiologists on Twitter. In the popular imagination, the phrase has become shorthand for the end of the pandemic—a finish line that will suddenly cause the virus to subside and allow maskless normalcy to resume.

Yet given how relentless, unpredictable, and mutation-prone the coronavirus has proved to be, top researchers are beginning to say a more realistic expectation for the Covid endgame is a slow, gradual improvement, with many bumps and setbacks along the way. Potent vaccines like those from Moderna and the partnership of Pfizer and BioNTech have put the world in a far better position than six months ago. But the virus, which has killed 3 million and infected more than 140 million worldwide, with no signs of letting up, is likely to circulate for years to come. In other words, the end to the pandemic may only become clear in retrospect.

The idea behind herd immunity is tantalizingly simple. Once a certain percentage of people become immune through vaccination or infection—perhaps 70% to 85% for this particular virus—transmission becomes more difficult and the protective effect shields the broader population. The formula, 1-1/R₀, with R₀ being the average number of new infections thought to result from each case, requires basic algebra. But dig into the details, and this intuitive concept gets complicated quickly. “Everybody talks about herd immunity as this really important threshold, but it’s a really crude and sort of hard-to-estimate number,” says Nicholas Reich, a biostatistician at the University of Massachusetts at Amherst, who develops Covid forecasts by combining data from different research groups worldwide.

Humans aren’t cows. Far from being a simple fixed number, the population percentage needed for herd immunity can vary over time and from place to place depending on a wide range of factors, including how long immunity lasts, how people behave, what mitigation steps are in place, how quickly the virus mutates, and even the local climate. A look at past vaccination campaigns provides a sobering perspective on the slog ahead. Smallpox is one of the only major human viruses that has been officially eradicated. Rare cases of polio still occur in some countries. Even measles took years to fully subdue in the U.S. with potent vaccines.

Herd Immunity Is Humanity’s Great Hope, and It’s Proving Elusive

Whatever the theoretical number might be, it’s gone up in recent months with the rise of more-infectious variants, like the B.1.1.7 strain that now predominates in the U.S. Early in the pandemic, some optimistic studies, often cited by shutdown opponents, claimed that as little as 10% or 20% of the population becoming infected might lead to herd immunity. Mainstream estimates, initially around 60% to 70%, have trended up over time. Lately, U.S. government officials have shied away from citing a specific figure, emphasizing getting shots in arms quickly. “I would like to get people away from this concept of referring to something that is very elusive in its definition,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, at a White House briefing on April 12. A return to normalcy will be gradual, he’s emphasized.

There’s no question that widespread vaccination will blunt the pandemic, invigorate the economy, and allow more normal social activities. The numbers of deaths and severe cases have plummeted in Israel, which is leading the world’s race to immunize its population. But in the bigger and more diverse U.S., vaccines have become a politically polarized issue. While more than a quarter of the population is fully immunized, vaccine hesitancy may stymie fuller uptake. (Republicans and White evangelical Christians are most likely to say they will abstain, polling shows.) Vaccine supply is already outpacing demand in parts of the country.

Herd Immunity Is Humanity’s Great Hope, and It’s Proving Elusive

“As we move forward, we’ll have pockets where people will not have been vaccinated, by race, by income, and by religion, unfortunately,” says Ali Mokdad, an epidemiologist with the Institute for Health Metrics and Evaluation in Seattle. “Automatically we will see outbreaks among these, or infections much higher among these groups, and we’ll never reach herd immunity.” Globally the vaccination campaign has barely begun, with enough doses administered to cover just 6% of the world population, according to Bloomberg’s vaccine tracker. Those doses are concentrated in a couple of dozen rich countries. “Our flank is exposed,” says Saad Omer, director of the Yale Institute for Global Health, who notes far too little has been spent on a worldwide rollout, given that viruses pay no attention to borders. “It boggles my mind to think we can have an immune fortress in America and learn to live with the risk of variants being imported.”

Recent vaccine setbacks won’t help. Many countries have limited use of AstraZeneca Plc’s adenovirus vaccine after it was linked to rare blood clots. The U.S. distribution of Johnson & Johnson’s shot is on pause while authorities investigate a handful of similar clotting events linked to that vaccine. Doctors advising the Centers for Disease Control and Prevention plan to meet on April 23.

In the long term, the biggest wild card is variants, especially those that reduce vaccine efficacy. After the spectacular vaccine trial results late last year, “I had thought we would see pretty much a normal lifestyle by the second half of 2021,” says virologist David Ho, who heads the Aaron Diamond AIDS Research Center at Columbia University, where he’s been studying variants. Although he still expects steady improvement, the explosion of variants has “decreased my optimism” for a quick conclusion to the pandemic.

Herd Immunity Is Humanity’s Great Hope, and It’s Proving Elusive

The rainforest city of Manaus, Brazil, shows how difficult the virus will be to wrestle into submission without high vaccination rates. The city was hit hard early in the pandemic, and one study found that 76% had been infected by October. That should have put it in herd immunity territory. But in December a deadly new wave struck, driven by the P.1 variant, which suggests there were people who got sick twice. “You cannot understand Manaus without some reinfections,” says Ester Sabino, an infectious disease researcher at the University of São Paulo. The best vaccines are thought to provide stronger protection than natural immunity.

In an optimistic scenario, the coronavirus may run out of ways to keep changing. But the complicated medium-term picture likely means a bumpy and zigzagging path to normalcy. It will involve relaxing restrictions in some places while grappling with new ways to live in a world where the virus is still out there. Eventually the coronavirus could become something more like influenza, steadily mutating in a way that necessitates a never-ending series of booster shots. Or SARS-CoV-2 could settle down and turn into another common cold.

With so many possible outcomes, researchers say the best strategy is to focus on what’s most under our control: ramping up vaccinations. “From an immunology and vaccine perspective, it is pretty simple: Vaccinate as many people as possible,” says Shane Crotty, a professor at the La Jolla Institute for Immunology. “Everybody is better off if you can vaccinate 90% of the population.”
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