A World Health Organization expert on Tuesday clarified that “modelling studies have put the extent of asymptomatic transmission to about 40%”, reported The Guardian. This came a day after Maria Van Kerkhove claimed that asymptomatic transmission was “very rare”. However, Van Kerkhove still maintained that real world data suggested it could be a rare event.

During a live question-answer session on social media, Van Kerkhove said she was not referencing the modelling studies when she made the claim on Monday. Van Kerkhove, who is the technical head of WHO’s Covid-19 team, said her comments during Monday’s press briefing were based on two-three studies following up the contacts of asymptomatic people and unpublished data.

“I used the phrase “very rare” and I think that it’s a misunderstanding to state the asymptomatic transmission globally is very rare,” she said on Tuesday. “What I was referring to was a subset of studies. I was also referring to some data that isn’t published.”

Asked about asymptomatic transmission, Van Kerkhove said, “We don’t have the answer yet.” She admitted that asymptomatic transmission happened, but scientists are still unsure about its frequency.

Van Kerkhove went on to explain that some people who appear to be infected but asymptomatic are in fact pre-symptomatic. “They may not register that they are sick,” she added. She said the other factor was how long people are infectious before they develop symptoms and how infectious they are in the days before they are visibly ill. “It is not only who is transmitting to others, but when,” she said. “The data is very preliminary.”

WHO Executive Director Mike Ryan also backed Kerkhove. He said the global health body welcomes discussion if “people think we are straying from evidence”. He added: “We were misinterpreted, perhaps because we didn’t use the most elegant words.”

Both Ryan and Kerkhove said understanding of Covid-19 was still an evolving science.

A study from China had earlier estimated that 44% patients had contracted the virus from an asymptomatic person, reported The Indian Express. “We observed the highest viral load in throat swabs at the time of symptom onset, and inferred that infectiousness peaked on or before symptom onset… Disease control measures should be adjusted to account for probable substantial pre-symptomatic transmission,” the study, published in Nature Medicine on April 15, had said.

In India, the testing strategy includes asymptomatic direct and high-risk contacts of a confirmed case. “I don’t think our testing strategy should change,” ICMR researcher Tarun Bhatnagar told The Indian Express. “We don’t know what data it is that WHO has seen… I also don’t know when WHO says asymptomatic whether they are making a difference between true asymptomatic and pre-symptomatic.”