When Could the United States Reach Herd Immunity?

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With the vaccine rollout underway and coronavirus cases declining after a dark winter surge, it may seem as though the end of the pandemic is in sight. In reality, how soon could we get there?

One answer lies in herd immunity, the point when enough people are immune to the virus that it can no longer spread through the population. Getting there, however, depends not just on how quickly we can vaccinate but on other factors, too, like how many people have already been infected and how easily the virus spreads.

An estimate for the path to herd immunity

20%40%60%80%100% of population immuneTotal immunity Fully vaccinated Dec.Jan. 2021Feb.MarchAprilMayJuneJulyAug.Sept.Oct.Nov.Dec.

This chart shows the current path to herd immunity in the United States, based on a model developed by PHICOR, a public health research group. It looks at the number of people who have been fully vaccinated and combines that with an estimate of the number of people who have been infected and have recovered to measure total immunity.

When the orange line crosses into the blue area, that means we have entered the herd immunity range. The exact threshold for herd immunity for the coronavirus is unknown, but recent estimates range from 70 percent to 90 percent.

At first, this looks like pretty good news — under these assumptions, we could reach herd immunity as early as July. But a lot could happen between now and then. The speed and uptake of vaccination, and how long immunity lasts are big factors. The rise of new virus variants and how we respond to them will also affect the path to herd immunity.

In most scenarios, millions more people will become infected and tens or hundreds of thousands more will die before herd immunity is reached.

What if we speed up vaccinations?

More than 15 million people have been fully vaccinated, and the U.S. is currently administering about 1.7 million shots per day. Some experts say we could nearly double that pace by April as new vaccines are approved. (Because the current vaccines require two doses spaced weeks apart, the number of people fully vaccinated each day is smaller.)

The more people we vaccinate, the faster we could reach the threshold for herd immunity.

CHOOSE A SCENARIO 

Maintain current pace

Increase to 3 million per day

Increase to 5 million per day

If the pace immediately increases to 3 million shots per day, we could reach the herd immunity threshold by May. In that time, 90,000 people could die from the virus.

20%40%60%80%100% of population immuneTotal immunity Vaccine immunity May Threshold reachedJuly Threshold reachedDec.Jan. 2021Feb.MarchAprilMayJuneJulyAug.Sept.Oct.Nov.Dec.

It’s important to note that the orange line for total immunity relies on an estimate of who has already been infected, including people who have immunity from undiagnosed cases. We can be more certain of reaching herd immunity when the pink line for vaccinations crosses into that range. But in a scenario where a new but less effective vaccine arrives, we might not reach the threshold through vaccination alone.

And the model comes with some other caveats. Much is still unknown about how long immunity from vaccines will last, or how well the vaccines will protect against new variants of the virus. The estimates also assume that the vaccine prevents infection rather than just reducing the severity of coronavirus symptoms.

“There are still key pieces of missing information that could substantially affect what may happen to the pandemic over the ensuing months,” Dr. Bruce Y. Lee, a professor at the City University of New York School of Public Health & Health Policy who leads the research effort for PHICOR, said. “Should many people lose immunity over the next several months after having recovered from infections, that would make many more people susceptible to the virus again.”

Some experts argue that reducing deaths and severe illness is a better and more achievable goal than full herd immunity, and ramping up vaccinations is still the best way to do that.

What if we relax social distancing?

Measures like wearing masks and social distancing have proven to be effective in slowing the spread of the virus. As more people get vaccinated, it’s tempting to ease up on those precautions.

While this means we would reach herd immunity sooner because more people would get sick and develop antibodies, that speed would come at a grim cost.

CHOOSE A SCENARIO 

Keep measures in place

Ease up in
the spring

End precautions now

If we end restrictions in April, we could reach the herd immunity threshold by June. But in that time, 170,000 more people could die from the virus.

https://www.nytimes.com/interactive/2021/02/20/us/us-herd-immunity-covid.html