Aggressive control measures appear to have halted the first wave of coronavirus in areas outside Hubei province in China, research suggests.

But scientists warn there is a substantial risk of the virus being reintroduced from abroad.

They say that with economic activity increasing, real-time monitoring of Covid-19 transmissibility and severity is needed to protect against a possible second wave.

The study published in The Lancet estimates that in regions outside Hubei, the average number of cases generated by a single infected individual during the outbreak fell after lockdown measures were introduced.

It adds that this number has since remained below one, suggesting the pandemic has shifted from one that is expanding rapidly to one that is slowly shrinking.

Mathematical modelling suggests premature lifting of control measures is likely to lead to transmissibility exceeding one again, resulting in a second wave of infection.

Researchers say their findings are critical for countries in early stages of lockdown.

However, the study did not specifically examine the effect of each intervention, or which was most effective in containing the spread of the virus.

Professor Joseph T Wu from the University of Hong Kong, who co-led the research, said: “While these control measures appear to have reduced the number of infections to very low levels, without herd immunity against Covid-19, cases could easily resurge as businesses, factory operations and schools gradually resume and increase social mixing, particularly given the increasing risk of imported cases from overseas as Covid-19 continues to spread globally.

“Although control policies such as physical distancing and behavioural change are likely to be maintained for some time, proactively striking a balance between resuming economic activities and keeping the reproductive number below one is likely to be the best strategy until effective vaccines become widely available.”

The data suggests the confirmed case fatality risk – the probability of dying among confirmed cases of Covid-19 as officially reported – outside Hubei was 0.98%.

This is almost six times lower than the 5.91% in Hubei and varies substantially among different provinces,

“Even in the most prosperous and well-resourced mega cities like Beijing and Shanghai, healthcare resources are finite, and services will struggle with a sudden increase in demand,” said senior author Professor Gabriel M Leung, from the University of Hong Kong.

On January 23 stringent restrictions were put on the movement of people and goods after the outbreak in Wuhan.

Since February 17, restrictions have been gradually relaxed in several provinces.

“We are acutely aware that as economic activity increases across China in the coming weeks, local or imported infection could lead to a resurgence of transmission,” said co-lead author Dr Kathy Leung, from the University of Hong Kong.

The researchers acknowledged their study had limitations, including that the estimated reproductive numbers were based on the reported number of confirmed cases, and the time and dates of symptom onset were unavailable for some provinces.

Also, a limited number of simulations for relaxing control measures were done, and did not specify which interventions or public responses to the outbreak might correspond to each of these scenarios.