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

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

Likewise, early 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聽the view that natural COVID-19 immunity may be short-lived 鈥� as little as 12鈥�18 months, the researchers note.

Associate Professor Kathryn Glass, an epidemiologist at the , 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.

鈥淲e don鈥檛 yet have solid data on the durability of vaccines, nor people鈥檚 natural immune response to the virus. Until we have data on that, it鈥檚 hard to know whether vaccine recipients聽and people that have already been infected聽will contribute to herd immunity,鈥� she said.

鈥淲e 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%.鈥�

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.

鈥淚f the 70% of people that are immune to COVID-19 are still able to spread the virus, that won鈥檛 help us achieve herd immunity,鈥� she said.

鈥淚t鈥檚 likely that those with underlying immunity will experience minimal viral shedding 鈥� but again we don鈥檛 know for sure if that鈥檚 true.鈥�

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

鈥淚f mutations outsmart existing vaccines, then herd immunity will grow evermore elusive,鈥� said Assoc Prof Glass.

鈥淗owever, 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 鈥� as quickly as possible.鈥�

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

鈥淰accines will stop most people from getting really sick from the virus, so it鈥檚 still important to proceed with the rollout,鈥� she concluded.

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