Many have advocated the approach, which aims to establish widespread immunity within a population by allowing the virus to spread and infect people.
But WHO chief Tedros Adhanom Ghebreyesus said the policy was "scientifically and ethically problematic".
“Allowing a dangerous virus that we don’t fully understand to run free is simply unethical. It’s not an option,” Dr Ghebreyesus said during a virtual press briefing on Monday.
“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.”
Dr Ghebreyesus insisted that “never in the history of public” has herd immunity been adopted as a strategy for responding to a serious outbreak.
His warning comes after an international group of scientists called on governments to allow young and healthy people to return to normal life, thereby building up natural immunity, while protecting the most vulnerable.
The proposal was criticised as “grotesque”, and it later emerged that several supposedly expert signatories of ‘the Great Barrington declaration’ were fake names.
"Herd immunity is achieved by protecting people from a virus, not by exposing them to it," said Dr Ghebreyesus.
The WHO head added that seroprevalence tests – where the blood is tested for antibodies – suggested that just 10 per cent of people globally have so far been exposed to the virus.
“The vast majority of people in most countries remain susceptible to this virus,” he Dr Ghebreyesus said. “Letting the virus circulate unchecked therefore means allowing unnecessary infections, suffering and death.”
He also pointed to the lack of understanding around the body’s response to Covid-19, as well as the long-term conditions that many people have developed as a result of exposure.
Maria Van Kerkhove, WHO’s technical lead on the virus, told Monday’s briefing that it was estimated that 0.6 per cent of people who contract Covid-19 die from the disease.
To date, the virus has killed more than one million people and infected 37.5 million since it first emerged in China late last year.
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