Before a population reaches herd immunity, an inflection point must occur — the point where there’s enough immunity that cases begin to decline consistently. Aleksandar Nakic/Getty Images
  • Before a population can reach herd immunity, an inflection point has to occur — the point where there’s enough immunity, through vaccination and previous infection, that cases begin to decline consistently.
  • Experts suspect the United States hit an inflection point in late April.
  • Cases may continue to steadily decline, especially as vaccination rates increase.

When we talk about building enough immunity in the population to blunt the spread of COVID-19, we usually refer to herd immunity.

Herd immunity prevents community transmission of disease when about 70 percent of the population is immune (either through vaccination or previous infection).

But before a population reaches herd immunity, an inflection point must occur — the point where there’s enough immunity that cases begin to decline consistently.

Israel, for example, hit an inflection point in early March when about 40 percent of the population was given the first dose of the COVID-19 vaccine.

Some experts think the United States hit its inflection point in late April when, similarly, 40 percent of the population received at least one dose. Cases have been declining since.

“Inflection point is defined as the point at which cases start falling after a certain rate of vaccination reached in the population,” explained Dr. Monica Gandhi, an infectious diseases specialist with the University of California, San Francisco.

From what we have observed in certain countries with high vaccination rates, once an area hits an inflection point, cases continue to decline steadily.

Real-world data shows the vaccines are extremely effective, and breakthrough cases are rare. Evidence also suggests that people who previously contracted the coronavirus are well-protected.

When there are fewer people who can contract the coronavirus because of increasing immunity, cases will drop, hospitalizations will plummet, and deaths will essentially disappear.

In Israel, cases and hospitalizations began declining when about 40 percent of the population had received at least one dose. And once cases began to dip, the decline sped up.

The United Kingdom hit its inflection point quickly.

“The U.K. was on a pretty strict lockdown when the vaccines were rolling out and also had a higher level of illness than other countries in Europe, which may have led to higher rates of natural immunity,” Gandhi said.

It looks as if the United States hit its inflection point in late April, Gandhi said. Since then, day after day, cases related to COVID-19, hospitalizations, and deaths have been decreasing.

“We seemed to have reached that inflection point around April 24, when cases starting coming down at a 40 percent first-dose vaccination rate in the U.S. as well,” Gandhi said.

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The inflection point will vary per state and even county depending on the level of natural immunity in the area, Gandhi said.

In areas that were hit hard by COVID-19 and therefore have higher natural immunity levels, fewer people need to get vaccinated to reach an inflection point. In areas mostly spared by COVID-19, more people will need to get vaccinated to hit that infection point.

Also, there may have been many asymptomatic cases of COVID-19, says Jennifer Horney, PhD, a disaster epidemiologist and founding director of the University of Delaware’s epidemiology program. “We don’t know exactly what our total coverage is, it could be higher than we think,” she said.

The inflection point will also fluctuate depending on an area’s local vaccination rate.

In Michigan, cases recently hit record highs. “There were lower surges in the winter, and so cases went up before coming down, likely due to a lower rate of natural immunity,” Gandhi explained.

Now, about 55 percent of people 16 and older in Michigan have received their first dose, and cases are finally declining for the second week in a row.

Though the country as a whole appears to have hit an inflection point, we may still see upticks in certain regions around the United States.

We could see “upticks in areas where there are lower rates of vaccination, and there have continued to be higher levels of community spread,” said Horney.

Growing evidence shows the vaccines work well against the variants.

During Israel’s vaccination campaign, the B.1.1.7 variant first identified in the United Kingdom gained a foothold, but the vaccine won out.

“Despite increasing prevalence of B.1.1.7 in Israel over the mass vaccination campaign — with a recent report showing >95 percent of the strains were B.1.1.7 later in MarchTrusted Source — higher rates of vaccination will eventually allow the reaching of the inflection point, despite variants,” Gandhi said.

As cases continue to decline, the risk that new variants will surface and spread also drops.

“If we can continue to reduce case counts, then we can also stop the spread of new strains at the same time,” Horney said.

Experts are optimistic that cases, hospitalizations, and deaths will continue to trend downward but recognize that we aren’t in the clear quite yet.

Cases have dropped significantly since this past winter, but the country still sees about 35,000 cases a day. We are now on par with where cases were at last September.

“The vaccine does seem to be effective, but the virus can still find susceptible people in the population who have either chosen not to be vaccinated or haven’t yet become eligible for the vaccine,” said Horney.

Over the next few months, it will be crucial for more people to get vaccinated. As we learned from Israel, a high vaccination rate will protect the population.

Health officials will need to make the vaccine as widely available as possible, provide as much encouragement and data as we can, and provide incentives for vaccination.

“It’s important to remember we have continued investments to make,” Horney said. “It’s not over.”