Why herd immunity may remain out of reach worldwide

As Covid-19 surged last year, governments worldwide touted the hope of “herd immunity”, a promised land where the virus stopped spreading exponentially because enough people were protected against it. That is now looking like a fantasy.

The thinking was that the pandemic would ebb and then mostly fade once a chunk of the population, possibly 60% to 70%, was vaccinated or had resistance through a previous infection. But new variants such as Delta, which are more transmissible and been shown to evade these protections in some cases, are moving the bar for herd immunity near impossibly high levels.

Delta is spurring widening outbreaks in countries such as the US and UK that have already been walloped by the virus, and presumably have some measure of natural immunity in addition to vaccination rates of more than 50%. It is also hitting nations that have until now managed to keep the virus out almost entirely, such as Australia and China.

This month, the Infectious Diseases Society of America estimated that Delta had pushed the threshold for herd immunity to well more than 80% and possibly close to 90%. Public health officials such as Anthony Fauci have drawn controversy by shifting the goalposts over the past year, increasing the number of people who need protection before hitting herd immunity. Meanwhile, vaccine hesitancy and supply issues mean most countries will not get close to even the original numbers.

“Will we get to herd immunity? No, very unlikely, by definition,” said Greg Poland, director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minnesota.

Even a vaccination rate of as high as 95% would not achieve it, he said. “It is a neck-and-neck race between the development of ever more highly transmissible variants which develop the capacity to evade immunity, and immunisation rates.”

Future variants

Nature will not solve the problem, either. It is unclear how long natural immunity gained from surviving Covid-19 will last, and whether it will be effective at fighting off new strains. Future variants, including some that could evade immunity even more efficiently than Delta, raise questions about how — and when — this will be over.

“If it was as simple as getting the infection once means you are immune for life, that would be great, but I don’t think that’s the case,” said SV Mahadevan, director of South Asia Outreach at the Center for Asian Health Research and Education at Stanford University Medical Center. “That’s a troubling problem.”

Already there are signs that some people, and some places — such as Brazil and other countries in South America — are being battered a second time by newer strains.

Without herd immunity, the virus could linger for decades in some form, possibly forcing the world’s most powerful nations to adjust their diverging strategies on opening borders and economies.

Countries such as China that have pursued tight, Covid-19-Zero policies by attempting to wipe out every infection, may eventually have to consider a looser stance. Others such as the US and UK that have opened up despite a resurgence of the virus run the risk of wave after wave of infection.

Vaccines so far have not been the quick-fix some had hoped for. Israel, among the world’s most-vaccinated countries, has already started administering booster shots, amid evidence that the current immunisations are not offering the protection that was hoped. Last week, the US said Americans with weakened immune systems will get a third dose.

The most powerful vaccines, including the mRNA shots from Pfizer, BioNTech and Moderna, would make it easier to reach high levels of immunity since they are so effective. Yet breakthrough infections — cases in the immunised — are possible with even these shots. Other vaccines, including those made by China’s developers, AstraZeneca and Johnson & Johnson, may offer even less protection.

Herd immunity is a real thing, protecting much of the world against viral threats from the measles to polio. Scientists credit it for helping eradicate smallpox. Having herd immunity as a goal probably helped the world embrace measures such as wearing masks and social distancing. But it also created a false narrative.

“The focus on ‘herd immunity’ has, in my view, been quite damaging,” said William Hanage, an epidemiologist and expert in communicable disease dynamics at the Harvard TH Chan School of Public Health. “It presents people with an unrealistic vision of how the pandemic comes to a close and doesn’t account for the evolution of either the virus or the nature of disease in reinfections.”

Some countries learnt the shortfalls of herd immunity the hard way. UK Prime Minister Boris Johnson originally planned to use it as the primary approach to Covid-19, suggesting some of his constituents could “take it on the chin” with natural infections before the magnitude of the coronavirus’ force became apparent.

Others are now throwing in the towel, with Indonesia most vocally leading the way. The world’s fourth-most populous nation determined that it would be impossible to stop the virus even if everyone in the country was immunised. It is redoubling its efforts to promote mask wearing and social distancing, while continuing to boost the still-low vaccination rate.

Meanwhile, Singapore and Australia are easing cautiously towards reopening, promising they will do so as they hit sufficiently high vaccination levels. Populations in Covid-19-Zero nations usually have lower levels of natural immunity built through previous infections.

Individualistic approach

Despite evidence that it will be difficult or impossible to reach herd immunity, many public health officials unwilling to give up on it. Governments worldwide are focused on widening inoculation programmes.

Yet the individualistic approach of many countries, and vaccine shortages, are contributing to the global problem. The risk of the virus remains for everyone, as long as any nation is experiencing huge outbreaks.

The world is unlikely to put the pandemic behind it until 2022 at the earliest, experts say. That target could be pushed back if the virus mounts another metamorphosis to become even more transmissible or even better at evading resistance.

There is hope for new vaccines and other approaches that could stop transmission more dramatically, but none of those are in human trials yet. It will be a few years before they are a real possibility.

Instead, the virus has a high likelihood of remaining entrenched globally, causing outbreaks that are hopefully mitigated partly by vaccinations, masking and other public health-driven interventions.

“Delta is not something we will be able to eradicate,” Hanage said. “Even Alpha would have been hard. However with sufficient immunity, ideally achieved by vaccination, we can expect it to become a much milder illness.”

The Spanish flu of 1918 shows how Covid-19 may play out, the Mayo Clinic’s Poland said. It is likely that variants will continue to emerge, forcing the use of boosters or routine immunisations, targeted to the newer strains.

“Then, if we are lucky, what is likely to happen is that this will become something more akin to influenza, where we will always have it,” Poland said. “It’ll become more seasonal, just like the coronaviruses that are already circulating, and we will just have to keep immunising.”

Bloomberg News. More stories like this are available on bloomberg.com.