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.
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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.
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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.
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“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.
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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 .
“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.”
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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.
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