People underestimate coronavirus at their peril, health officials have warned, as they voiced concerns the Omicron variant is being dismissed by some as “mild”.

Data from South Africa is still only in its early stage and countries must act now in the face of a potentially “large wave of cases” of the faster-spreading mutation, Dr Mike Ryan of the World Health Organisation (WHO) said.

His warning was echoed by the organisation’s director-general Dr Tedros Adhanom Ghebreyesus, who said efforts must be made to jab the unvaccinated as well as continuing with measures like mask-wearing and hand-washing.

It came after Dr Angelique Coetzee, chairwoman of the South African Medical Association, told the Commons Science and Technology Committee she agrees with a new study from the South African Medical Research Council suggesting that Omicron may be 29% less severe than the first wave of infection that swept the country.

She said “we don’t have all the answers”, but the clinical picture so far is that people are mostly suffering mild illness from Omicron.

But the WHO has warned against relying too heavily on early data.

Dr Tedros told a press briefing: “We’re concerned that people are dismissing Omicron as mild.

“Surely, we have learned by now that we underestimate this virus at our peril.

“Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems.”

Dr Ryan said that while scientific monitoring of the variant is ongoing, “we need to be ready to deal with what is likely to happen, which is a large wave of cases, which may or may not be more or less severe but which will in themselves generate pressure on the health system”.

The executive director of the WHO’s health emergencies programme cautioned that “health systems are weaker now than they were a year ago, in reality”, as multiple waves of Covid-19 have swept through countries.

He added: “Unfortunately, sometimes you can get up after the first punch , but it’s very hard to get up after the second and third, and that’s the difficulty.”

He said leaders at WHO will be “the happiest people in the world” to be able to say in two or three weeks from now that “this is a much milder disease, everything is fine”.

But he warned against making assumptions, as he said: “That’s not how this virus has behaved up to now, that’s not our experience through the three waves of this pandemic.

“So I think the idea is to act now in the real world while we collect the data to understand exactly what this virus is capable of.”

No-one who is offered a booster should feel guilty about accepting their third dose, Dr Ryan said, when asked about the UK’s ramping up of its rollout.

He said countries can protect their own citizens while also helping to address global vaccine inequity.

He said: “There are governments out there, like the UK, who are trying their best to deal with the priorities of their own people, and supporting, very strongly, international efforts to create equity in vaccine distribution.”

Dr Tedros said the priority must remain “to vaccinate the unvaccinated, even in countries who have better access to vaccines”, and said jabs must be combined with other health measures.

Countries can “take the heat out of the transmission by doing the simple things” such as avoiding crowded spaces and ensuring good ventilation, Dr Ryan said.