Despite what science or the failed coronavirus strategy in Sweden tell us, people continue to entertain herd immunity as a possible strategy for ending the Covid-19 pandemic.
Herd immunity is a fantasy
Published: September 19, 2020
It seems Trump meant herd immunity, rather than “herd mentality,” but no matter — the line of thinking he was apparently trying to invoke goes like this: if Americans let SARS-CoV-2, the virus that causes Covid-19, run amok, then eventually enough people will be immune (around 70 to 90%) that the virus will no longer pose a threat to the population.
I’ve previously written about how reckless and ineffective this line of thinking is, and many experts have agreed. The strategy would cause a catastrophic number of deaths in the US. And Sweden, which took a lax approach to coronavirus restrictions, is still far from the herd immunity threshold. Thanks to researchers using genomic sequencing techniques, we now know that people can be reinfected with Covid-19 — a fact that should be the final nail in the coffin of any ill-conceived hopes for herd immunity.
Human coronaviruses in general have not been known to engender herd immunity. They’ve infected us like clockwork for decades — many of us many times over the course of our lives. Until recently, it was unclear whether this would be the case for SARS-CoV-2. Scientific case studies of two patients reinfected with Covid-19 — one in Hong Kong and the other in the United States (which is still undergoing peer review) — have since answered that question. The only one that remains is how often reinfection occurs.
What might shed some light on this issue is a collaborative, multiyear study led by researchers from the Netherlands and Kenya on the human coronavirus NL63 (HCoV-NL63), a member of the coronavirus family that isn’t lethal but is endemic to populations worldwide. Like other nonlethal human coronaviruses, which likely infect us all at least once in our lifetime, HCoV-NL63 causes what we’d call a common cold. Three key findings from this study, which was published in 2018, can help inform our understanding of the Covid-19 virus SARS-CoV-2 and its ability to reinfect us.
The study was carried out across homes and hospitals in Kilifi County, Kenya, where researchers tracked the circulation of multiple coronaviruses throughout the community over a span of six years. The first relevant finding was that some patients weren’t reinfected just once, but twice. One person even contracted the virus four times. This tells us that immunity to HCoV-NL63 was short-lived — so much so that some were reinfected within as little as three months.
The second significant finding was that a surprising number of patients had higher viral loads upon reinfection than they did during primary infection. For most people, both the amount of virus in their body and the severity of their symptoms decreased from one infection to the next. But for some patients in the NL63 study, the opposite was true. This seems to be the case for the 25-year-old man in Nevada who was reinfected with Covid-19.
In fact, researchers in the NL63 study postulated that the high antibody levels in some of the Kenya patients increased the likelihood of infection rather than preventing or mitigating it — an outcome that defies the more general expectation that a strong immune response would mean greater protection from reinfection. These cases suggest that no one who contracts a coronavirus, regardless of the nature of their primary infection, is totally safe from reinfection.
The third significant finding is that in six years, herd immunity to HCoV-NL63 was never acquired, and the virus continued to persist for the entire duration of the study. Granted, endemic human coronaviruses aren’t as contagious as SARS-CoV-2, but they’ve been infecting people around the world for decades and always manage to come back. Even if SARS-CoV-2 does become an endemic human coronavirus like HCoV-NL63, we won’t be rid of the dangers it presents to vulnerable populations — unless we have a widely used vaccine that can give us the protection our natural immune response cannot.
It is in our best interest to acknowledge the risk of reinfection — rather than holding out hope for herd immunity — when thinking of the scientific and public health interventions we’ll need in the months and years to come. We must place the bulk of our efforts on developing and equitably distributing a vaccine that is not only safe and effective, but attendant to the possibility of reinfection. We must redouble our efforts to stage evidence-based public health interventions until a vaccine is available. Otherwise, it will be a long time before we’ve seen the last of Covid-19 — and not long at all before we’re bested by another lethal coronavirus once more.