At a news briefing last week, World Health Organization officials called the pursuit of such a herd immunity strategy “very dangerous.”
“If we think of herd immunity in the natural sense of just letting the virus go, it’s very dangerous,” said Maria Van Kerhove, WHO technical guide for the covid-19 pandemic. “A lot of people would die.”
Soumya Swaminathan, the WHO’s chief scientist, said the focus on controlling transmission through public health measures while scientists develop vaccines should be the main strategy. She said that “there really was no infectious disease that could be controlled only by allowing natural immunity”
The United Kingdom pursued such a strategy early, but abandoned it when officials saw the consequences. Sweden, which has followed a similar strategy, has been heavily criticized by health officials and infectious disease experts as reckless: the country has among the highest levels of infection and mortality in the world.
But the idea of ”herd immunity” continues to attract attention in some neighborhoods: Conservative TV presenter Laura Ingraham wrote on Twitter that pursuing herd immunity is “the only practical way forward.”
How exactly does herd immunity work?
Herd immunity occurs when enough people become immune to a disease that the virus cannot transmit to new hosts, which slows its spread. The two main ways to achieve herd immunity are through natural immunity – when you get the virus and your body develops antibodies that protect it from future infection – or through vaccinations.
Depending on the virus, there is a certain percentage of the population that must achieve immunity before herd immunity can be reached – called the ‘immune herd threshold’.
For example, measles – a particularly contagious disease – only slows down after about 95 percent of people become immunized.
It remains unclear how much of the population must be infected with the new coronavirus to reach this threshold. Estimates range from 20 to 80 percent. As scientists have learned more about the virus, some have narrowed their estimates to between 40 and 70 percent.
Suminanat said that given the coronavirus’s susceptibility, she estimates that about 65 to 70 percent of the population will have to become infected to achieve herd immunity.
Reaching such a threshold would “take a very long time,” she said, noting that seroprevalence studies that measure human antibodies to the virus show that, on average, only 5 to 10 percent of people worldwide have them. This percentage is higher in some cities, such as New York, where many are infected. (A new study suggests that people without antibodies may still have T-cell immunity from earlier contact with other coronaviruses, but this has not been proven.)
Swaminathan said that these low levels of antibodies in most places mean that “the majority of the world’s population is still susceptible to this virus, which means the infection can go on and on and on in waves and so the best way to achieve this kind of immunity of the population would be through a vaccine. “
Why is pursuing a herd immunity strategy a deadly business?
Proponents of herd immunity talk about segregating and thus protecting the elderly, nursing home residents and others who are more likely to die from the virus, while allowing the virus to spread among the young. But growing evidence shows that young people who work away from home or rush to bars and restaurants when the states relax infect their more vulnerable elders, especially family members.
A similar strategy is complicated in the United States by the fact that most young and middle-aged people here have higher levels of obesity, heart and lung disease, and other health problems that make them more likely to have serious or fatal cases of the disease.
Exactly how many people would die before a population reaches immunity depends on a number of complex variables, but even elementary calculations with the back of the shell show that it is likely to be significant.
A population of 328 million in the United States may need more than 2 million deaths to reach the 65 percent herd immunity threshold, assuming the virus has a 1 percent mortality rate, according to an analysis by The Washington Post. Even if the herd’s immunity threshold and mortality rate are at the lower end of current estimates – with a 40% need to be infected and a 0.5% mortality rate – the country can still expect 656,000 deaths to achieve herd immunity, almost four times more than the country has already suffered.
Why America can’t just sit and wait for herd immunity
It is also unclear whether people recovering from covid-19 have long-term immunity to the virus or can be reinfected. This can complicate the way herd immunity can be achieved. Scientists are still learning exactly who is vulnerable to the disease. From a practical point of view, it is almost impossible to sufficiently isolate the people most at risk of dying from the younger, healthier population, according to public health experts.
Harvard epidemiologist Mark Lipsic says that even when a community or nation reaches herd immunity, there will still be cases. “That doesn’t mean the cases stop,” he said. “It just means they’re starting to slow down.”
“We would all like to know how far we are from the end of this,” Lipsic said. “But no matter what the exact threshold, it’s largely an academic issue because we’re not close to herd immunity.”
“No desirable thinking will make that go away,” said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota. “We can’t wait for herd immunity. We can’t even wait for a vaccine. We must do everything possible to reduce transmission. We need plans and national actions. We need a vision. This is not something we can keep kicking along the way. We cannot continue to talk about the next month or two months from now. We need action now. “
Yasmin Abutaleb and Harry Stevens contributed to this report.