With hundreds of researchers around the world trying to find a vaccine for the coronavirus pandemic, a World Health Organisation envoy has cautioned that a vaccine might never appear at all.

Dr. David Nabarro serves as a professor of global health at Imperial College London and raised the possibility in a new interview with CNN.

“There are some viruses that we still do not have vaccines against,” Nabarro told the outlet.

“We can’t make an absolute assumption that a vaccine will appear at all, or if it does appear, whether it will pass all the tests of efficacy and safety.”

“It’s absolutely essential that all societies everywhere get themselves into a position where they are able to defend against the coronavirus as a constant threat, and to be able to go about social life and economic activity with the virus in our midst,” he added.

It’s been a challenge to develop vaccines for common rhinoviruses and adenoviruses, which similar to the coronavirus, can cause cold symptoms, according to Dr. Nabarro.

“You have high hopes, and then your hopes are dashed,” Nabarro said.

“We’re dealing with biological systems, we’re not dealing with mechanical systems. It really depends so much on how the body reacts.”

However, most experts are hopeful a COVID-19 vaccine will emerge as the deadly bug does not rapidly mutate.

More than 90 COVID-19 vaccines are underway in various stages by research teams in companies and universities worldwide.

Human trials have also begun in an Oxford University-developed vaccine that has protected six rhesus macaque monkeys from “heavy quantities” of the pathogen.

This article originally appeared on Over60.