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The Omicron variant may be more transmissible, but increased transmissibility does not necessarily mean it will cause less severe disease, the World Health Organization (WHO) said in a briefing on Wednesday.

Omicron has been detected in 57 countries, with more still expected, WHO officials said, with its global spread and large number of mutations causing concern. However, it is too early to draw conclusions, given the lack of data on clinical disease.

Mike Ryan, MD, executive director of WHO's health emergencies program, said that while a virus will adapt to become more transmissible, the idea that a more transmissible virus is a milder virus is "a little bit of an urban legend."

"The outcome of whether a virus is less severe is much more random," he said. "I don't expect in general for viruses to become milder, it can happen randomly, but it's not a process that as a disease becomes more transmissible, it becomes milder."

If mild disease is allowed to go unchecked, it can generate pressure on the healthcare system, he added.

"Hope is not a strategy and we need to be very, very careful" when talking about severity, he said.

However, COVID-19 boosters for all are not the answer either, said Kate O'Brien, MD, WHO's director of the Department of Immunization, Vaccines and Biologicals, adding that the WHO's Strategic Advisory Group of Experts on Immunization (SAGE) recently met to do an "intensive review" on boosters, and that preliminary data indicated the vaccines are "holding up" for the severe end of the disease spectrum, while there have been "further reductions in performance" on the mild end.

"No matter which way you look at it, primary doses always outperform booster doses for those who are at risk," she said. "Primary doses to those who have not been vaccinated have to be the priority."

Soumya Swaminathan, MD, WHO's chief scientist, added that boosters make sense for vulnerable populations, such as those with comorbidities, but "boosters are unfortunately not the solution to this."

WHO officials referenced several early reports on lab data showing a reduction in neutralizing antibodies for the Pfizer vaccine, but pointed out that neutralizing antibodies are only one component of immunity. More clinical data are needed, they said, especially in countries with different populations, such as South Africa, which has a younger population. They also noted early reports on increased risk of reinfection with Omicron, but stressed these were also preliminary.

O'Brien said that while vaccine performance may be affected by the variant, that does not mean vaccines will become "ineffective entirely." She added that herd immunity is more important than ever, noting that people "can't be relying on the concept that other people being vaccinated are going to protect those in the community who choose not to be vaccinated."

By that same token, Ryan said that while preliminary indications are that Omicron is transmitting more efficiently than Delta, that "does not mean the virus is unstoppable," rather that it has become "better adapted to exploiting the connections between us."

That means non-pharmaceutical interventions, such as masking and social distancing, are particularly important, WHO officials said. WHO Director-General Tedros Adhanom Ghebreyesus, MBBS, PhD, called on countries to immediately increase their surveillance, including testing and sequencing capabilities, and to submit more data to the WHO's clinical platform.

"The steps countries take today ... will determine how Omicron unfolds," he said. "Don't wait until hospitals fill up. Act now ... Any complacency will cost lives."

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