Speaking on Tuesday, Dr. Tedros Adhanom Ghebreyesus warned that discussions of reaching “herd immunity”—the point at which a population is considered protected from a virus—had misunderstood what the concept entails.

“Herd immunity is a concept used for vaccination, in which a population can be protected from a certain virus if a threshold of vaccination is reached,” he said. In measles, the threshold was 95% of a population, while in polio, the threshold is 80%, he noted.

“In other words, herd immunity is achieved by protecting people from a virus, not by exposing them to it,” Ghebreyesus said. “Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic. It is scientifically and ethically problematic.”

That’s because not enough is known yet about immunity to the coronavirus, and it was still not clear how strong or lasting an immune response is, he said. Some people have also been reinfected, he added.

Meanwhile, the vast majority of the population has still not been exposed to the virus, with surveys indicating that in most countries, less than 10% of the population has been exposed. He also referred to those who have experienced long-lasting symptoms from COVID-19.

“Allowing a dangerous virus that we don’t fully understand to run free is simply unethical,” said Ghebreyesus. “It’s not an option.”

The WHO continues to advocate for a targeted approach to containing the virus, relying on well-functioning systems to trace, test, and isolate cases of COVID-19 before they spread.

However, many countries—including the U.S. and the U.K.—are struggling to implement even the most basic infrastructure required to “test and trace,” including building capacity for tests and persuading people to register where they go and whom they interact with. Political division, too, has undermined decision-making on how to approach containing the virus.

The concept of “herd immunity” appears to be making a comeback as cases rise and governments struggle to control the virus while often resisting full-scale lockdowns of the type seen in the spring.

But the concept already became more mainstream at the start of the pandemic, as the U.K. in particular briefly appeared to be pursuing the strategy before being forced into a U-turn by public outcry and scientific advisers, who warned the strategy would result in huge numbers of deaths, particularly of the elderly and most vulnerable.