Few of us considered ourselves part of a herd before, but the concept rooted in livestock management made it disconcertingly clear that we, too, are animals at the mercy of a microbe. Then herd immunity became a political flashpoint for those who argued it was better to let the virus rampage than impose lockdowns.
Now, it's mostly invoked as the pandemic finish line — the point when so many people have immunity from vaccination or infection that the virus will fizzle out like sparks that can't find sufficient tinder to sustain a flame.
Or, as King County's top health official describes it: "some sort of magical threshold."
But Dr. Jeffrey Duchin doesn't put much stock in magic. He's one of a growing number of experts who doubt herd immunity against the novel coronavirus will ever be achieved — and who say it doesn't really matter.
"It would be great if we reached that threshold," says Duchin, public health officer for Public Health — Seattle & King County. "I think it's very questionable that we will, and I also think we don't need to achieve true herd immunity to return to a normal lifestyle."
Herd immunity essentially requires vanquishing the virus except for occasional flare-ups that can be quickly quenched. But there are several obstacles, foremost among them the emergence of new variants that are more infectious, possibly more deadly, and better able to dodge immunity.
A more realistic goal, Duchin and others say, is preventing serious illness and death by vaccinating as many people as possible, as quickly as possible. The virus will almost certainly continue to circulate, but it won't be nearly as deadly.
It would, in essence, become more like influenza — a virus we coexist with and have somewhat tamed through long exposure and regular vaccination, says Deborah Fuller, a UW Medicine microbiologist and vaccine expert.
"Instead of herd immunity, maybe we should be talking about getting back to where we're not going to have to worry about dying or having our loved ones in the hospital or not being able to hug our grandparents anymore," she says. "We live with flu. It still kills 30,000 people a year, but it doesn't kill 500,000 people."
Vaccine vs. the variants
That's a shift for Fuller, who initially thought the hard work was over when the first vaccines were shown to be about 95% effective in preventing symptomatic COVID-19. Some models estimated as few as 60% of Americans might need to be vaccinated in order to reach herd immunity.
More contagious variants "kicked the can" further out of reach, Fuller says. Vaccine coverage needs to be higher to counter the variants' ability to jump more easily from person to person. Estimates of proportion of the population that would have to get shots began creeping up and now range as high as 90% — though no one knows for sure.
One of the most pressing questions is whether the variants will significantly erode vaccine effectiveness, says Washington State Epidemiologist Dr. Scott Lindquist.
The state recently bought high-speed genetic sequencers to boost surveillance of the mutants and is also looking for signs the variants are escaping immunity. "If someone who's vaccinated gets the disease, I want to know if it's one of the variants breaking through," Lindquist says.
Evidence remains sparse, but the B.1.1.7 variant, first detected in the U.K., seems almost as vulnerable to vaccines as the original virus, while variants first detected in South Africa (B. 1.351) and Brazil (P. 1) seem more adept at evading antibodies induced by several vaccines, including Moderna's and Pfizer's.
Natural immunity is also breaking down against some of the variants. In South Africa, people who were infected with the original strain are now falling sick again from a variant. With 75% of residents infected in an early wave, the Brazilian city of Manaus seemed like a good candidate for herd immunity. But the virus is surging again, and reinfection with the P. 1 variant appears to be on the rise.
A rapidly evolving virus makes herd immunity even more difficult to achieve, Duchin says. The pathogen will always be one step ahead and regular boosters may be necessary to keep up.
The encouraging news is that vaccine-induced immunity is more robust than natural immunity, says Dr. Larry Corey, co-director of the national COVID-19 vaccine trials network based at the Fred Hutchinson Cancer Research Center. And most of the vaccines remain highly effective — even against the variants — in their most crucial function: preventing hospitalization and deaths.
"This is the relevant first metric for the success of the vaccine program," Corey says. "We need to get the vaccine into people's arms and educate people about the importance of this ... and stop talking about herd immunity."
"Herd immunity" origin
Corey has never been comfortable with the emphasis on herd immunity, especially once it became politicized. "I feel the concept has been used too imprecisely," he says.
It was originally coined for strategies to eliminate "contagious abortion" from bacterial infection in sheep and cattle. "Abortion disease may be likened to a fire, which, if new fuel is not constantly added, soon dies down," a pioneering veterinarian wrote in 1918, advising ranchers to breed only cows that were immune and avoid introducing new animals.
Herd immunity in humans rarely — if ever — develops from natural infection. Measles, the best-known example, was brought under control by vaccination alone. At 95% coverage, people who can't get shots because of age or health conditions are protected by the herd effect.
But measles rarely mutates. The same vaccine has remained effective for decades.
The measles shot also generates what's called sterilizing immunity, preventing the virus from replicating in the human body. With the COVID-19 vaccines, immunity is more of a continuum than an on-off switch, says UW Medicine immunologist Marion Pepper. And that can be another problem for herd immunity. Even though they're protected from severe illness, vaccinated people may still get mild infections and pass the virus to others.
New evidence, including from Israel where 40% of the population is fully vaccinated, suggests that may be less of a concern than originally feared — though more data are needed. "I think people are pretty optimistic that these vaccines are reducing transmission, and that's important," Pepper says.
But other impediments to herd immunity remain, including uncertainty about how long vaccine protection lasts. At least 15% of American adults say they will never get the shots, and there's no vaccine yet for children and most teenagers, who comprise more than 20% of the country's population, Duchin points out.
While it's likely some regions, especially in wealthy countries, will eventually reach very low levels of transmission, the threat of emerging variants will remain until vaccination is global, says Ali Mokdad, of the UW's Institute for Health Metrics and Evaluation.
Countries like the U.S. and the U.K., which have locked up more than enough vaccine should consider donating the excess to poorer nations, he says.
"We have a moral obligation, and it's in our national interest to protect everyone else in the world so we can be protected."
"Dry tinder" for variants
The next few months will be a race between the vaccines and the variants, with the outcome likely to determine whether there's a fourth wave of disease this spring caused by the U.K. variant, says The Hutch's Dr. Joshua Schiffer. His team's modeling suggests Washington is particularly vulnerable precisely because the state has done such a good job in controlling the virus.
Only about 8 to 15% of Washingtonians have been infected so far, which leaves a lot of "dry tinder" for the variants to spark, Schiffer cautions.
Lindquist, the state epidemiologist, isn't too worried. More than 1.7 million vaccine doses have been administered in the state, and rates are picking up. If variant cases start to spike, he's optimistic residents will double down on masking and social distancing and flatten the curve again.
Trevor Bedford, a Fred Hutch computational biologist whose pandemic analyses have sometimes seemed prophetic, said recently he expects transmission of the B.1.1.7 variant to continue increasing in Washington and the U.S., causing "more of a wave" in April or May than otherwise expected. But by summer, he says the virus should fall to very low levels.
But another wave is possible in the fall as the variants that originated in South Africa and Brazil become more common.
Fuller, the microbiologist, advises humility.
"This virus always surprises," she says. "There's always something new around the corner."
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