How soon will we reach herd immunity for COVID-19?

Achieving herd immunity for COVID-19 may be a challenge through current vaccination programs, research from the Imperial College London has found.

While existing vaccines, like Pfizer and BioNTech’s, have demonstrated high (95%) levels of efficacy — 15% more than what experts say is needed to reach herd immunity for COVID-19 — the duration of vaccine protection is not yet known.

Likewise, early data has revealed fading antibody titres (levels) in those that have recovered from the virus. Although antibodies form only one part of the immune response, studies on other coronaviruses support the view that natural COVID-19 immunity may be short-lived — as little as 12–18 months, the researchers note.

Associate Professor Kathryn Glass, an epidemiologist at the Australian National University, says that, in order to achieve herd immunity, longer-term protection from both vaccines and previous COVID-19 infections is required.

Additionally, given the high reproduction number of SARS-CoV-2 (the pathogen that causes COVID-19), an estimated 70% of the global population would need to become immune simultaneously.

“We don’t yet have solid data on the durability of vaccines, nor people’s natural immune response to the virus. Until we have data on that, it’s hard to know whether vaccine recipients and people that have already been infected will contribute to herd immunity,” she said.

“We know that mRNA vaccines are really good at protecting against disease. But if a vaccination program takes three or more years to roll out among the global population, and vaccines only offer 18 months of protection, then it will be quite difficult to reach that milestone of 70%.”

Moreover, it is not yet known whether those with individual immunity are still able to transmit the virus. If so, a 70% immunisation target would be meaningless, Assoc Prof Glass added.

“If the 70% of people that are immune to COVID-19 are still able to spread the virus, that won’t help us achieve herd immunity,” she said.

“It’s likely that those with underlying immunity will experience minimal viral shedding — but again we don’t know for sure if that’s true.”

An additional concern is the threat of viral mutation, which may impact the durability of natural and vaccine-induced immunity, some experts have warned.

“If mutations outsmart existing vaccines, then herd immunity will grow evermore elusive,” said Assoc Prof Glass.

“However, viral mutations do take time and it could take many years for the virus to become that smart. Hence the urgency to vaccinate as many people as possible — as quickly as possible.”

Although no solid conclusions can be drawn yet, Assoc Prof Glass emphasised that a vaccination program is still warranted.

“Vaccines will stop most people from getting really sick from the virus, so it’s still important to proceed with the rollout,” she concluded.

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