- Many factors can influence how soon the United States will reach herd immunity for COVID-19, but many health experts expect the end of the pandemic is in sight.
- It's theoretically possible the United States could reach herd immunity by the end of spring.
- But new SARS-CoV-2 variants and spotty vaccine availability make this unlikely, according to some experts.
Also known as community immunity, this is the point at which enough people are immune to a virus — through vaccination or natural immunity — that the virus no longer easily spreads through a population.
The precise threshold for herd immunity for SARS-CoV-2, the coronavirus that causes COVID-19, is unknown. But recent estimates range from 70 to 90 percent.
That means a majority of people would need immunity to block the spread of the virus through a community.
Herd immunity isn't just an abstract calculation made by virologists and epidemiologists. It has implications for how soon public health restrictions such as physical distancing and mask mandates can be safely lifted.
To put it another way: How soon will life return to “normal”?
Some health experts see longer return to ‘normal’
Many health experts are optimistic that the end of the pandemic in the United States is in sight, although it’s not clear what the transition to “normal” will look like.
Some experts are more optimistic than others.
“I expect Covid will be mostly gone by April, allowing Americans to resume normal life,” wrote Dr. Marty Makary, a surgeon and a professor at the Johns Hopkins School of Medicine and Bloomberg School of Public Health, in a commentary in The Wall Street Journal.
He points to the sharp drop in COVID-19 cases in recent weeks as a sign that parts of the country are nearing, or have reached, herd immunity.
This decline in cases is “in large part because natural immunity from prior infection is far more common than can be measured by testing,” he wrote.
He estimates that the number of people who have recovered from a coronavirus infection — and likely have some immunity to the virus — is 6.5 times the 28 million confirmed cases.
That would mean that about 55 percent of Americans have natural immunity.
If you combine this with the 150 million people that Makary estimates will be vaccinated by the end of March, that brings the country close to the herd immunity threshold.
Not every health expert, though, agrees that the country will be open for business by April.
Many factors affecting herd immunity in flux
Dr. Eric Topol, director and founder of the Scripps Research Translational Institute, was even more critical of Makary’s Wall Street Journal commentary.
“Just wondering if WSJ has any fact checkers,” he wrote on Twitter.
Topol has several concerns about the commentary, including Makary’s estimate of how many people have natural immunity due to past SARS-CoV-2 infection.
Other research suggests that the level of natural immunity in the United States may be lower than Makary’s calculation.
The Centers for Disease Control and Prevention (CDC) estimates that at the end of December, 83.1 million people had had an infection — about 25 percent of the population.
More recently, Columbia University researchers estimated that by the end of January, 36 percent of people in the United States had had an infection — or 118 million people.
Topol also doubts that the United States will hit 150 million vaccinations by March.
President Joe Biden set a goal of 100 million people vaccinated within the first 100 days of his administration, which stretches through the end of April. He later upped the goal to 150 million.
Based on an analysis of CDC vaccination data by NBC News, the country is on track to hit the more ambitious goal by April 29.
If you take the Columbia University researchers’ estimate of the extent of natural immunity in the country, millions of people are still susceptible to this new coronavirus.
The country has already paid a high price for this level of natural immunity. To date, more than 500,000 people have died of COVID-19 in the United States.
If public health measures are relaxed too soon — before enough people can be vaccinated — the country could see another spike in cases, hospitalizations, and deaths.
Variants could throw off projections
“My modeling suggests that it is increasingly unlikely that the US will reach the immunity levels required for theoretical herd immunity in 2021,” he wrote on Twitter.
He said his projection is based on several new developments over the past month.
One is the high number of people who won’t be vaccinated anytime soon — either the third of Americans who say they won’t get vaccinated, or children under age 16 who aren’t yet eligible for a vaccine.
Gao said another development that could affect herd immunity is new SARS-CoV-2 variants that may lower the effectiveness of COVID-19 vaccines.
Data suggests that B.1.351, a variant first identified in South Africa, may reduce protective antibodies elicited by some vaccines.
If the vaccines offer less protection, more people will need to be vaccinated in order for the country to reach herd immunity.
Topol is also concerned about the impact of more transmissible variants.
“[Makary] turns a blind eye to variants,” he wrote on Twitter, “particularly B.1.1.7, which has accounted for peak pandemic surges, hospitalization, and deaths in Israel, UK, Ireland and Portugal, and [is] now seeded throughout the US.”
Other factors can also influence how long it will take the country to reach herd immunity, such as the duration of immunity — from the vaccine or after infection — and whether the vaccine prevents people from transmitting the virus to others.
Scientists are still studying both of these questions.
Christina Ramirez, PhD, a professor of biostatistics at the UCLA Fielding School of Public Health, said until we know the answer to these questions, we’ll have to keep some public health measures in place.
Vaccination is safest route to herd immunity
The New York Times and The Washington Post created interactive models that show how different factors can affect the path toward herd immunity — and the number of people who could die from COVID-19 under different scenarios.
These models highlight what public health officials have long been saying about COVID-19: The safest path to herd immunity is through vaccination.
Vaccines train the body to recognize and fight the coronavirus without causing COVID-19 or the long-term complications of infection so many “COVID long haulers” are experiencing.
Although Gao’s model has contributed to the herd immunity discussions, he cautions against focusing too much on reaching this threshold.
“Our goal should not be to reach ‘herd immunity,’ but to reduce COVID-19 deaths & hospitalizations so that life can return to normal,” he wrote on Twitter.
Ramirez agrees. She points out that the COVID-19 vaccines approved so far have all been very effective in reducing severe COVID-19 and deaths.
“Even with the variants, data shows that the new vaccines really work at reducing hospitalizations and deaths,” she said. “Variant or no variant, if you are at risk [for COVID-19], you should get the vaccine and not wait for herd immunity to protect you.”