LONDON — The head of the World Health Organization warned against the idea that herd immunity might be a realistic strategy to stop the pandemic, calling such proposals “unethical.”

WHO director-general Tedros Adhanom Ghebreyesus said at a press briefing on Monday that health officials typically aim to achieve herd immunity — where the entire population is protected from a virus when the majority are immune — by vaccination. Tedros noted that to obtain herd immunity from measles, for example, about 95% of the population must be vaccinated.

“Herd immunity is achieved by protecting people from a virus, not by exposing them to it,” he said. “Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak,” he said, calling the strategy “scientifically and ethically problematic.”

Tedros said that WHO estimates less than 10% of the population has any immunity to the coronavirus, meaning the vast majority of the world remains susceptible.

Tedros also noted countries had reported record-high daily figures of COVID-19 to the U.N. health agency for the last four days, citing surges in Europe and the Americas in particular.