Rumors have circulated online that California and Washington state had reached “herd immunity” for the coronavirus, with enough of the population becoming infected and developing antibodies that the virus can no longer spread.
Experts say that is unlikely.
“At the community level, there would not have been enough infections to really have enough umbrella of herd immunity,” Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN on March 31.
“If I had to put my nickel on it, we don’t have very high herd immunity in this population currently,” Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington, told Business Insider. “What we’ve done is reduce our transmission, and every model shows that if we open things up now, we will just have a rebound.”
That makes the next steps for the US tricky. Cities and states will likely have to open things up slowly, test widely to track the virus’s spread, and impose lockdowns again before new waves of infection grow too large. Through that process, experts hope that the population will build its immunity and limit the deaths that come with overwhelmed hospitals.
“Right now, we can’t develop herd immunity because there’s not a lot of people getting infected,” Halloran said. “We don’t know how much there is right now.”
Immunity in 50% of the population could starve out the virus
Halloran sees herd immunity as “a continuous thing,” which increases as more people get infected. But she said that “some people talk about herd immunity as a threshold: You get enough immunity in the population that you can stop transmission because there aren’t enough susceptible people left.”
That threshold, where herd immunity prevents the virus’s spread enough to kill it off slowly, is determined by a crucial measure called R0 (pronounced R-naught). R0 refers to the average number of people that one sick person goes on to infect.
The R0 for the coronavirus currently sits around 2 to 2.5, meaning that each infected person spreads the virus to an average of 2.2 others.
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However, as more people get infected, recover, and hopefully develop immunity, that number slowly inches downward. If the R0 dips below 1, that puts the pathogen in decline until it dies out.
Experts say that, given an R0 of 2 to 2.5, it would take at least 50% of the population developing immunity to make that happen for the coronavirus.
“If I get infected, when I’m interacting with some people, the chances of me running into a susceptible [person] is much lower because the people I’m running into already have immunity,” Halloran said. “I’d have to interact with a lot of people to find a susceptible [one], and that’s where the curve comes down.”
Only a small portion of the US population could be immune
On Sunday, Trevor Bedford, a researcher at Fred Hutch who has tracked the spread of COVID-19 through its genome, calculated a rough estimate of how many people in the US have recovered from the virus.
Since limited testing has been prioritized for severely ill patients, many mild cases of the coronavirus are likely slipping under the radar, leading to far more recovered people than reported.
“I’d guess that we’re catching between 1 in 10 to 1 in 20 infections as a confirmed case,” Bedford tweeted. “This would give 5-10 million infections in the US or about 2-3% cumulative prevalence. This is a long way from the 50% (R0 of 2) to 66% (R0 of 3) we’d need for herd immunity.”
Blood tests that can detect coronavirus antibodies are on the way, and they could offer a more accurate picture of how many people have been infected.
That wouldn’t necessarily tell us how many people are immune, though. It’s still unclear how much protection antibodies confer on people who have recovered from COVID-19, or how long their immunity lasts.
“People don’t understand the immunity to this particular virus. What we hope is if you get it once, you’ll be protected against it for at least a year,” Halloran said. “We don’t know that, but that’s what we hope.”
How to go forward without sufficient herd immunity
Eventually — most likely via a vaccine — communities or countries could achieve the herd immunity threshold. But experts say the US should be planning to get through the next two years of COVID-19 without it.
Halloran says we can make it until a vaccine is available by quickly identifying and isolating infected people, tracking down who they may have infected, and providing enough protective equipment for frontline workers.
“If we can mitigate or suppress infection, we would be living with some infection and probably some deaths, but it wouldn’t be overwhelming. The infection would sort of limp along in the population because we wouldn’t necessarily stamp it out, but it wouldn’t travel very fast,” Halloran said. “That’s a different concept than herd immunity.”
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