who: letting virus spread through herd immunity is "scientifically and ethically problematic"

this story was originally published on july 23, 2020 and has been updated.

• the difference between natural herd immunity and vaccinations
• canada is a far way from the population needed for herd immunity
• even if it were achievable, it’s unclear how long immunity to covid-19 lasts

the head of the world health organization says relying on herd immunity by exposing people to covid-19 would be “scientifically and ethically problematic.”in recent months, the words “herd immunity” seem to have become an argument for slowing the spread of covid-19. while it can be effective for stopping the spread of widespread illnesses through vaccination, such as seasonal influenza, natural herd immunity through exposing people will not stop the spread of the virus that causes covid-19.“never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic” said tedros adhanom ghebreyesus, director-general of the world health organization, at a regular press briefing earlier this week.“letting the virus circulate unchecked therefore means allowing unnecessary infections, suffering and death,” he said. it’s unknown how immunity to covid-19 works and whether it would last.less than 10 per cent of the world’s population has been infected with covid-19, according to who estimates, which leaves the “vast majority” susceptible.“we well understand the frustration that many people, communities and governments are feeling as the pandemic drags on, and as cases rise again,” said the who chief. “there are no shortcuts, and no silver bullets. the answer is a comprehensive approach, using every tool in the toolbox.”

what is herd immunity?

herd immunity occurs when large proportions of the population is immune to infection to the point that it stops disease from spreading from person-to-person. if enough of the population has already had the disease and is immune to it, then the disease can’t spread as quickly. this is especially vital for those who are vulnerable to disease, like newborns, the elderly or those immunocompromised.there are two ways to achieve herd immunity.natural herd immunity. natural herd immunity requires large parts of the population to contract the disease and become infected, which in turn causes each person’s immune system to build antibodies that prevent them from getting infected again. however, herd immunity also comes with the risk of serious health risks to those infected and requires everyone to become infected at the same time, which quickly overwhelms healthcare resources.vaccinations. vaccinations are a safer and more ideal path to herd immunity. vaccines create individual and herd immunity without causing illness or death. some examples of deadly contagious diseases that have been controlled through herd immunity via vaccines include smallpox, polio, diphtheria, and rubella. measles has made a resurgence recently due to low vaccination rates. researchers all over the world are currently working on a vaccine for covid-19, with more than 155 in development, 23 of which are currently in human trials.even in spain, one of the world’s most hard-hit countries, the number of people in the general population who have antibodies is very low. researchers wrote in a recent lancet study that “herd immunity is difficult to achieve without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems.” the study found only five per cent of the country’s population had developed antibodies to the novel coronavirus — a far cry from the overwhelming majority that would be needed to achieve herd immunity.to further complicate the way we understand herd immunity, the percentage of a population that needs to have antibodies varies by disease. for a disease that is as contagious and undetectable as covid-19, it would likely require great proportions. according to the mayo clinic, measles — another highly contagious illness — requires 94 per cent of the population to have immunity in order to stop the chain of transmission.

it’s still not clear how long immunity to covid-19 lasts

however, even if the majority of ontario’s population were to contract covid-19, herd immunity may not last.“it only works if the antibodies are fighting the disease,” says dr. emily seto, an assistant professor at the dalla lana school of public health.there is still not enough evidence to determine how immunity to covid-19 works — after all, the disease was only detected for the first time in the world six months ago. the virus may mutate, like the common cold or seasonal flu, which means even if you build immunity to it, you could get it again. or immunity may be linked to severity of case: the more severe the disease, the more antibodies, or one might not be immune to covid-19 at all after getting it. and if immunity is possible, we don’t know exactly how long it lasts.one pre-print study found antibodies gradually vanish after about two months, another study found the drop happened within two to three months.  another study published in the new england journal of medicine this week found that people who got mild illness from covid-19 and developed antibodies that provided immunity for just a few months. researchers said antibodies had a half-life of 73 days, which means half the antibodies were gone by then.but a drop in antibodies is expected, researchers say it’s normal that antibodies decline when the worst of an infection is over.  a new pre-print study published this week (it has not yet been peer-reviewed) found a slower decline in antibodies over the months.“while this cannot provide conclusive evidence that these antibody responses protect from re-infection, we believe it is very likely that they will decrease the odds ratio of getting re-infected, and may attenuate disease in the case of breakthrough infection,” the study authors write.

herd immunity raises ethical questions

“another question is [whether it is] ethical to let everybody get infected,” says seto. [herd immunity] comes at a very high cost. obviously, there are going to be high mortality rates and a high percentage of people will have severe side effects.”and then there is the moral cost. “it may never work because you could get it again,” she says. “you could never be immune to it. even if 70 or 90 per cent of the population gets it — we don’t know the threshold, but it has to be high — the people who get it may not be immune.”if we were to accept that 80 per cent of our population were to become immune, then there is only 20 per cent of people who the already-infected could give the disease to — meaning covid-19 won’t spread as quickly, which is the good part.“but that only kicks in if such a large fraction of people have had covid-19, [then] that means we’ve already failed if we think the goal is to keep the population from getting covid,” says jeffrey rosenthal, a statistics professor at the university of toronto. “herd immunity will only kick in when we’ve really failed.”in a previous interview with healthing, biostatistician ryan imgrund says even if official reports are missing one out of every 20 cases, covid-19 has only perforated 4.7 per cent of ontario’s population, which is far from herd immunity.last week, a new pre-print study (not yet been peer-reviewed) on sero-prevalence found less than one per cent of british columbians have antibodies as of may.researchers from the university of british columbia, the b.c. centre for disease control, lifelabs and public health scientists looked at the blood samples of 885 anonymous patients and found only four had antibodies.while having an anonymized study is good because people are less afraid to report, the sample may still be skewed because people are more likely to come to the hospital if they think they’ve been exposed, have some symptoms, or have a reason to come in, says rosenthal.“it needs to be randomized to be accurate,” he says. “the only way to really test what fraction of the population has been exposed is to do antibody tests not just on people feeling sick or coming to the hospital, but randomly selected people.”while it’s still unknown if being exposed to covid-19 means being immune to ever getting it again — and let’s assume there is immunity — there may be many more people who have had coronavirus but are unaware because they didn’t show any symptoms and weren’t tested. the number of people who were exposed to the coronavirus is likely higher than we know, says rosenthal.“but because the fraction of people who are known to have covid-19 is still such a small fraction of the population, even if the true number of exposed is quite a bit higher, it’s probably not high enough. if it’s only a few per cent, that has a very small impact on the reproduction number.”diana duong is a writer and editor at healthing. find her on twitter @dianaduo.dduong@postmedia.com