As a number of western nations, including the United States, wind down stay-at-home orders, the World Health Organization said on Monday that attempting herd immunity without a vaccine is a “dangerous” idea.
Herd immunity is generally achieved when enough members of a population have antibodies to a virus that the spread of a virus slows. Immunity can generally be attained be either getting infected or having a vaccine.
But as viruses mutate, antibodies lose their effectiveness. It’s also not fully known how long antibodies keep people immune from catching the virus. According to Johns Hopkins University, flu antibodies generally last less than a year, and antibodies for other types of coronaviruses range from a few months to a few years.
Michael Ryan, the World Health Organization’s Executive Director of Health Emergencies, did not mince words when the organization was asked on Monday about the concept of herd immunity.
"This idea that, 'well, maybe countries who had lax measures and haven't done anything will all of a sudden magically reach some herd immunity, and so what if we lose a few old people along the way?' This is a really dangerous, dangerous calculation," Ryan said.
“Humans are not herds,” Ryan added. “As such, the concept of herd immunity is generally reserved for calculating who would need to be vaccinated in order to obtain that same effect. I think we need to be very careful when we use terms around natural infections of humans because it can lead to a brutal arithmetic, which does not put people, life and suffering at the center of that equation.”
The WHO cited a number of serologic studies that are looking at the prevalence of the virus worldwide. While only one study has been fully peer reviewed, and a few others are in pre-review, the evidence is that few humans have attained antibodies, leaving a still vulnerable population despite a worldwide death toll of nearly 300,000.
"A very low proportion of the people that have been tested have evidence of antibodies," Dr. Maria Van Kerkhove, the WHO's technical lead for COVID-19, said. "The range is between 1 and 10%. It depends on the study. "
Dr. Van Kerkhove told reporters that it’s unknown exactly how much of a population needs to be infected for it to attain herd immunity, but “it certainly needs to be higher than what we’re seeing in these serologic prevalence studies,” she said.
Serologic studies test the general population for antibodies to the virus, and not for active cases. The studies can give scientists an indication of not only how susceptible a population is, but how potent and potentially deadly a virus has become.
“It is showing the proportion of people with significant clinical illness is a higher proportion of all those that have been infected because the number of people that have been infected in the total population is probably much lower than we expected,” Ryan said. "That means we have a long ways to go.”
Johns Hopkins University has published information on herd immunity.
“With some other diseases, such as chickenpox before the varicella vaccine was developed, people sometimes exposed themselves intentionally as a way of achieving immunity,” Johns Hopkins University professors Gypsyamber D’Souza and David Dowdy wrote. “For less severe diseases, this approach might be reasonable. But the situation for SARS-CoV-2 is very different: COVID-19 carries a much higher risk of severe disease and even death.”