Elusive herd immunity comes at the cost of equity, vaccine expert says

Reaching the elusive goal of herd immunity to COVID-19 is unrealistic and instead governments should focus on equitable distribution of vaccines, said Dr. Shabir Madhi, director at the vaccines and infectious diseases analytics research unit at the University of the Witwatersrand in Johannesburg, and co-director at the African Leadership in Vaccinology Expertise.

“I think as a scientific community, we really need to recalibrate what to expect of COVID-19 vaccines,” he said. “This magic threshold of herd immunity is extremely unlikely. All indications are that vaccines are not going to take us to that threshold by any stretch of the imagination.”

“I think the high-income countries pursuing that particular agenda, unfortunately, compromises access to vaccines in the low- and middle-income countries,” he added.

Herd immunity — a term rooted in animal husbandry — is when a certain threshold of immunity is reached in a population that even those without immunity receive levels of protection because the virus finds it difficult to infect enough people and continue its spread.

The goal of herd immunity has been at the forefront of public discussion since the onset of the pandemic — a collective aspiration of a time when life might return to pre-pandemic days. In those early months, there were suggestions that populations could reach this threshold by letting the virus run rampant through communities to create natural immunity.

In May 2020, Dr. Michael Ryan, executive director at World Health Organization Health Emergencies Programme, called this a “very brutal arithmetic” that would lead to preventable deaths and untold suffering.

The conversation has since evolved, and now herd immunity is largely used in the context of vaccines. The problem is that COVID-19 vaccines are only modestly effective at reducing transmission depending on the variants, although they are highly effective at preventing severe disease across all current variants of concern, Madhi said.

The percentage of the population needed for herd immunity against COVID-19 is unclear. And whatever it is, it's ever-changing, depending on the transmissibility of the virus. Data suggests the delta variant is between 40% to 60% more transmissible than the Alpha variant, and almost twice more transmissible than the strain originally found in Wuhan, China.

There's no “magic number” of what percentage of people in a population need to be vaccinated to reach herd immunity, Katherine O’Brien, director of the department of immunization, vaccines, and biologicals at the WHO, said during a recent press briefing.

The African Centres for Disease Control & Prevention has set out to vaccinate 60% of its population by the end of next year to achieve herd immunity. But Dr. John Nkengasong, the director of the agency, said at a recent press conference this figure was calculated before the emergence of the delta variant, meaning it's likely this percentage isn’t high enough.

For example, with measles, because the disease is so transmissible, there is a need to vaccinate about 95% of the population.

Studies on the ability of COVID-19 vaccines to prevent transmission are difficult and require studies on each variant and each vaccine, O’Brien said.

The goal of herd immunity is purely aspirational and when governments obsess over this, it comes at the expense of equitable access to vaccines, Madhi said. High-income countries have hoarded vaccines and immunized large portions of their populations, as African nations have only fully vaccinated less than 2% of the continent’s population.

“We need to, I believe, accept that COVID-19 is here to stay — the virus is not going to be eliminated,” he said.

All governments should instead focus on the reduction of severe cases and deaths amongst the most vulnerable people globally, with greater access to vaccines for low- and middle-income countries, Madhi said.

“For me, that's the only tangible goal. I think anything else is aspirational and would never be realized in a majority of countries, especially countries which have gotten almost no access to COVID-19 vaccines,” he said.

Israel’s experience should be a “wake-up” call that reaching herd immunity is not on the horizon, Madhi said. Despite having 63% of the population fully vaccinated, Israel has seen a rise in cases. More than half of the cases are among fully vaccinated people, who are still less likely to need hospitalization or die.

“This clearly demonstrates that even if you vaccinate close to two-thirds of your population … you are still going to get resurgences, with this evolution of variants that are more transmissible,” he said.

Countries with high levels of their population immunized should continue to expect to see new outbreaks of the disease, although the magnitude of these resurgences might become less.

“But across all of these resurgences, what we are likely to continue observing now is a nominal number of people ending up in hospital and dying of COVID-19,” he said.

And governments need to ensure vaccination campaigns are coupled with the promotion of other interventions that work — such as wearing masks, washing hands, and social distancing, O’Brien said.

Mutations of the virus are expected to continue. As more people receive the vaccine globally, the pressure on the virus to mutate increases as it works to escape its own destruction by vaccines, Madhi said.

“It's close to impossible for any single country to eliminate the virus and open its borders and not expect importation,” he said.

According to Madhi, in general, the legacy of vaccines has always been to protect against severe disease and death. “Anything else is more of a bonus,” he said.

First-generation COVID-19 vaccines largely target the spike protein. Next-generation vaccines would need to change this approach to protect against infection more effectively, he said, adding the plausibility “remains to be seen” and it is still an “aspirational goal.”

Madhi said the types of mutations currently taking place are specifically on the spike protein, which is not really affecting the T-cell immune response, which seems to be the major protection against severe disease.

“That sort of provides a window as to what vaccines are really able to achieve,” he said. “They work particularly well against severe disease, but not necessarily against small disease and against infection.”