|FROM ||Ruben Safir
|SUBJECT ||Subject: [Hangout - NYLXS] What It Would Take for Herd Immunity to Stop the
What It Would Take for Herd Immunity to Stop the Coronavirus Pandemic
Brianna Abbott and Jason Douglas
The concept of herd immunity is at the heart of global vaccination
efforts and discussions about next steps in fighting the Covid-19
pandemic and bringing back economies.
For the pandemic to stop, the coronavirus has to run out of susceptible
hosts to infect. Herd immunity occurs when enough people in a population
develop an immune response, either through previous infection or
vaccination, so that the virus can’t spread easily and even those who
aren’t immune have protection.
To reach herd immunity for Covid-19, public-health authorities estimate
that around 60% to 70% of a given population would need to develop an
immune response to the virus. Some epidemiologists and mathematicians
now say herd effects might start to kick in before that point, at
perhaps closer to 50%, suggesting potential protection could be achieved
Still, infectious-disease experts adamantly warn against the notion of
trying to reach herd immunity to the coronavirus without a vaccine, as
the costs on human life would be staggering and it likely wouldn’t
happen soon, if at all.
Even with a vaccine, there will still be barriers to achieving herd
immunity. “It’s a continuous process,” said Saad Omer, director of the
Yale Institute for Global Health. “You could start seeing [an effect]
before that threshold, but the other issue is there might still be
outbreaks at a smaller level.”
“It’s not like when herd immunity is achieved, we’re not going to have
Covid-19,” he said.
Estimates vary widely for where the herd-immunity threshold lies because
researchers use a variety of statistical techniques to account for
differences in individuals’ likelihood of spreading the virus.
The herd-immunity threshold is mathematically dependent on the
infectiousness of the virus, or how many individuals each infected
person goes on to infect—called the basic reproductive number, or R0.
Scientists calculate the herd threshold using that number and an
equation formulated almost 100 years ago by two pioneering Scottish
Measles, for example, is extremely contagious. It has a basic
reproductive number between 12 and 18 and a herd threshold of 90% to
95%. For SARS-CoV-2, the basic reproductive number is estimated at
around 2.5 to 3, implying a herd threshold of 60% to 70%.
But the classic equation makes an assumption—that everyone is equally
susceptible to infection and has the same chance of bumping into every
other person, like molecules of gas in a bag, said Justin Lessler,
associate professor of epidemiology at Johns Hopkins Bloomberg School of
“Which of course is ridiculous. It’s not how the world works,” Dr.
Lessler said. The equation gives you a good target for vaccine coverage,
but it doesn’t capture the dynamics of an epidemic moving naturally from
person to person, he said.
In reality, people live in clusters, interacting only with certain
individuals. Age, job, social network and even individual biological
responses to infection all affect a person’s place in the spidery web of
Essential workers, for instance, are more likely to get infected than
people able to limit interactions with others. If those who are more
likely to transmit the virus developed immunity early on, whether
through infection or vaccine, it would be possible to start seeing a
reduction in transmission earlier, epidemiologists say.
Infectious-disease experts warn against the notion of trying to reach
herd immunity to the coronavirus without a vaccine.
Photo: Michael Nagle/Xinhua/Zuma Press
Reflecting these real-world effects in disease models can shift the
estimated herd immunity boundary. One group of researchers estimated
that threshold for Covid-19 could be as low as 10% to 20%, though many
epidemiologists say that is unlikely. Other modelers have estimated it
at around 40% to 50%. Christopher Murray, director of the Institute for
Health Metrics and Evaluation at the University of Washington, said his
group estimates the herd-immunity threshold at between 50% and 80%.
If lower estimates for herd immunity are correct, then governments
should consider policies to shield the elderly and other at-risk groups
from the virus while relaxing restrictions on everybody else, said Paul
McKeigue, professor of genetic epidemiology and statistical genetics at
the University of Edinburgh, in Scotland.
But most infectious-disease experts strongly advise against that, as it
isn’t clear how governments would shield more vulnerable people from the
easily spread pathogen. Those in lower-risk groups can become seriously
ill and, on rare occasions, die as a result of the disease. Doctors are
just beginning to understand long-term health effects.
“If we’re talking about letting the disease run its course [and] infect
large numbers of people, essentially what we’re saying is the
public-health system has failed to do its job,” said Nadia Abuelezam, an
infectious-disease epidemiologist at Boston College. “We’ll have a lot
of death, and we’ll have a lot of morbidity.”
Communities, including some neighborhoods in New York, and even
individual families that have been hard-hit by the virus likely do have
an elevated level of protection. “But it’s a dangerous thing to base
your strategy on,” said Dr. Omer. “There’s still enough tinder to be
How far along the candidate vaccines are
Each line represents one vaccine candidate. Colors represent the
technologies each vaccine uses to produce an immune response in the
body. The length of the line represents the stage of testing.
Note: Data as of Sept. 17
Source: World Health Organization
Most places aren’t likely near the point where everyone is protected,
even as confirmed global infections surpass 30 million and deaths top
And even if subpopulations do develop a level of protective immunity,
the virus can easily jump to unexposed pockets, especially if
restrictions are relaxed and people potentially start to socialize and
travel more, epidemiologists say.
In places where cases have slowed, behavioral changes such as
mask-wearing and social-distancing are the more likely explanations for
reduced transmission, said Natalie Dean, a biostatistician who
specializes in infectious diseases at the University of Florida.
Sweden, for instance, which didn’t implement an aggressive lockdown,
still has bans on gatherings of more than 50 people, among other
restrictions and voluntary guidelines.
A preliminary survey from the Swedish Public Health Agency in May showed
that only 6.7% of people from ages 20 to 64 and 2.7% of those from ages
65 to 95 had antibodies. Sweden also has the highest per capita death
rate of its Nordic neighbors.
“At this point, I would not bank on herd immunity in most areas on this
planet,” Florian Krammer, a professor in vaccinology at the Icahn School
of Medicine at Mount Sinai in New York, said during a panel recently at
the online Aspen Ideas health festival.
Israel became the first developed country to impose a second nationwide
lockdown amid rising Covid-19 cases. As businesses are forced to close
ahead of major Jewish festivals, WSJ's Dov Lieber meets restaurant
owners who worry they may never reopen. Photo: Amir Cohen/Reuters
Another wrinkle: Infectious-disease experts aren’t sure how long
immunity to the virus might last, or how often re-infections might
occur. Past exposure to other coronaviruses might provide some
protection, though any effects of that are unclear.
For diseases such as measles, mumps and polio, herd immunity was
achieved through vaccination campaigns. Many say the Food and Drug
Administration could authorize a Covid-19 vaccine by year-end, but the
rollout will likely be staggered well into 2021, incrementally reducing
the susceptibility of the population and building up resistance.
And even when one does potentially become available, there are growing
concerns that some Americans won’t want it.
How much of the population needs to get vaccinated to help achieve herd
immunity also depends on a potential vaccine’s effectiveness. The FDA
has said it would authorize a vaccine with 50% efficacy or better. Some
vaccines could require more than one dose to build up a robust response.
The coronavirus is unlikely to disappear completely, public-health
experts say. More likely, there will be outbreaks in vulnerable pockets
that authorities will work to control, similar to measles, or it could
circulate similarly to influenza.
But public-health precautions such as mask-wearing can beat back the
virus and, coupled with widely administered vaccines, safely help a
level of normalcy return, health authorities say.
“With a combination of a good vaccine together with good public-health
measures, we may be able to put this coronavirus outbreak behind us,”
Anthony Fauci, director of the National Institute of Allergy and
Infectious Diseases, told The Wall Street Journal’s CEO Council on
Wednesday. “There will be an end to this, and we will be able to get
back to normal.”
Write to Brianna Abbott at brianna.abbott-at-wsj.com and Jason Douglas at
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