Will we ever reach herd immunity?

Since the beginning of the COVID-19 pandemic, health officials, government leaders, and media reports have discussed reaching herd immunity—when enough people have become immune to a virus (either by vaccination or natural infection) that the virus can’t meaningfully spread.

Some countries seemed to be approaching herd immunity by the end of this summer. In the United Kingdom, for example, 81 percent of the population age 16 and up had been vaccinated as of mid-September.

Yet as the virus mutates, new variants may put that goal out of reach. Even with its high vaccination rate, the U.K. continued to see high numbers of new COVID-19 cases.

We take herd immunity for granted with diseases like measles: In the United States, when outbreaks do start (typically via someone who has traveled from a country where it remains endemic), they spread through a small number of unvaccinated people before fizzling out. “Large” outbreaks are measured in the hundreds of cases, and no American is known to have died from measles since 2015. This makes sense, because 92 percent of Americans have received the measles vaccine.

Early in the pandemic, some people hypothesized that the threshold for herd immunity to COVID-19 might be much lower, with the Swedish ambassador to America predicting the United States would reach herd immunity as early as May 2020. Those hypotheses continued through last year, with our coverage from September 2020 noting that estimates for the herd immunity threshold ranged from 20 percent to 70 percent. All of those, however, predated the virus’s more-contagious delta variant.

Delta transmits more easily between people: An August study in the Journal of Travel Medicine found that it roughly doubles infectivity compared with the original strain. Second, Nature reports that during an outbreak in southern China, almost three-fourths of delta transmissions appeared to happen before symptom onset. In other words, before a person began to feel sick and knew to stay home, he or she could have passed the virus on.

Vaccines also appear to be less effective in preventing minor infections from the delta variant, which may be enough to pass COVID-19 on to others. Over time, even vaccinated people and those who had previously contracted the coronavirus may begin to lose their immunity. This has given rise to debates about whether the COVID-19 vaccines should join the list of vaccines needing three—or more—shots. (For instance, the diphtheria-tetanus-pertussis vaccine involves a series of five doses, and the polio vaccine involves four.)

Will we reach herd immunity, then, with COVID-19? Sir Andrew Pollard, who heads the Oxford Vaccine Group, says no: He told the British All-Party Parliamentary Group (APPG) in August that “this current variant, the delta variant, will still infect people who have been vaccinated and that does mean that anyone who is still unvaccinated at some point will meet the virus.”

The participants in the APPG forum emphasized vaccines still protect their recipients: While the shots don’t reliably prevent mild cases of the delta variant, they remain strong against more severe infections. This might sound odd, but it makes sense: A vaccine’s goal is to teach the immune system to recognize a given invader and make antibodies quickly. An invader that moves more quickly might get a foothold—that’s the mild infection—but ramping up antibody production without delay can prevent that mild infection from becoming serious. This makes a major difference: A Public Health England study found the vaccines continued to offer excellent protection against hospitalization, including from the delta variant. (That explains why the vast majority of hospitalized patients in the current COVID-19 wave are unvaccinated.)

Evidence now points to the virus becoming a permanent, rather than temporary, part of our lives—albeit with variants raising the stakes for people who avoid vaccines. Pollard may be correct that we will fall short of true herd immunity, but it isn’t a binary all-or-nothing phenomenon. Each person who receives a vaccine before exposure, each person who doesn’t get sick after exposure, and each person who gets a mild case instead of more severe case of COVID-19, represents one more step toward the end of the public health emergency.