Will we ever reach herd immunity to Covid?
In May 2020, we and other scientists predicted that many regions of the world might never reach the herd immunity threshold for Covid-19 – the point at which enough people are immune to infection that transmission begins to slow down.
This remains true today, even as vaccines have become accessible in wealthy nations and many people have built up immunity through vaccinations, boosters and previous infections.
The herd immunity threshold was commonly misunderstood as a universal target to hit early in the pandemic. But the threshold has always been changeable: it depends on how transmissible the pathogen is, and the behavioural and immunological characteristics of the population in which it is spreading – how much they mix and how easily they are infected.
For example, if a virus is very transmissible, able to more readily infect people, or the population is very densely packed and mobile, a large proportion of the population will need immunity to halt its spread. Conversely, if a virus is less transmissible or a population doesn’t mix often in large groups, fewer people need to be immune to slow the virus down. In each case, the precise herd immunity threshold would be different.
The original Sars-CoV-2 virus strain had a herd immunity threshold that was estimated at 60-75%. But more recent and more transmissible variants, such as Delta and Omicron, likely have thresholds upwards of 80-90% – and that’s assuming everyone who has been vaccinated or previously infected is fully protected against future infection.
In fact, for Omicron, existing immune protection against infection and transmission is much lower (although protection against severe disease is still thought to be very high), so even with 90% of the population vaccinated we would not likely see Omicron transmission dying out.
Behavioural changes such as wearing masks, working from home and avoiding large gatherings can slow the virus’s spread. But as these interventions are lifted or varied across populations virus transmission will again accelerate, raising the herd immunity threshold compared to that of a more cautious population. At the same time, social inequities may exacerbate the disproportionate toll Covid has already had on marginalised communities by raising the herd immunity threshold locally, for example if people live in crowded housing.
Last year, some scientists suggested that we were approaching the herd immunity threshold through a combination of vaccination and infection. But a year after Covid-19 vaccinations began, we are seeing the largest spikes in cases to date in many regions, including places where population immunity from infection and vaccination is quite high. The highly infectious Delta and Omicron variants have driven recent surges through their high transmissibility and ability to partially evade immunity, making a much larger fraction of us susceptible to infection again.
This experience underscores that we have yet to – and likely will never – reach the herd immunity threshold despite the remarkable success of vaccines. At the same time, we must not resign ourselves to endless, explosive outbreaks.
We now find ourselves in the awkward transition to Covid-19 endemicity. In a prescient paper published in Science in January 2021, Jennie Lavine and her co-authors predicted that, like the other human coronaviruses that now cause the common cold, Covid-19 would eventually transition to a mild infection that continues to circulate at lower levels in the human population as people become exposed and immunised at early ages. In other words: an endemic virus.
Once Covid-19 is endemic, infection-blocking immunity would rapidly wane so that the virus would still spread easily but immunity against severe disease would last longer, causing infections and (primarily mild) disease to shift into younger age groups yet to be exposed or immunised; instances of severe disease, which primarily affects adults without prior immunity, should decline.
However, we’re not at that point yet. The authors cautioned that behavioural interventions to slow the spread were still necessary during the transition to endemicity to avoid overwhelming surges in hospitalisations and deaths, not to mention the symptoms of long Covid, which is now estimated to affect 1.3 million people in the UK alone. Vaccination (and periodic boosting) remain important for blunting the worst outcomes as we transition to endemicity.
It’s important to remember that the buildup of immunity within a population has benefits for everyone, even when the herd immunity threshold cannot be reached. Severe disease is dramatically reduced, preserving healthcare resources. When vaccinated people do become infected they may be infectious for less time and have lower viral loads, reducing onward transmission. Protection is multiplicative when vaccinated people mainly come in contact with each other, as the odds of becoming infected and passing on that infection are both reduced.
It is still the case that the uncontrolled spread of Covid-19 in undervaccinated regions could lead to the evolution of new variants that continue to cause disease. So far, Omicron typically seems to cause less severe disease than previous strains of the virus – and competition between variants (for example Omicron out-competing Delta in many regions), may work in our favour. But even milder variants still have the potential to inundate hospitals if they are highly transmissible. Rapidly vaccinating and boosting the global population, especially those facing severe inequities in vaccine availability, and combating the misinformation that hinders vaccine uptake remain some of the best ways to slow the emergence of new variants, and the uncertainties and setbacks they bring.
We can prepare for endemic Covid-19 now by improving access to vaccines, high-quality masks and tests, and requiring them in public settings where risk is high. We should invest in research on and distribution of treatments to prevent severe outcomes and social support for people grappling with Covid-19’s long-term effects. We must also strengthen our public health infrastructure to prevent future pandemics, establish regulations for safer work environments and mitigate health disparities that have been exacerbated by the pandemic. Covid-19 is not going away, but we can manage it with smart policy and collective action.
Erin Mordecai is an associate professor of biology at Stanford University. Mallory Harris is a PhD candidate at Stanford University, where she studies infectious disease