Officials want to know roughly when the outbreak will peak so they can prepare hospitals.Credit: SOPA Images/LightRocket/Getty

Coronavirus infections in China continue to swell by thousands a day, prompting epidemiologists to estimate when the outbreak will peak. Some suggest the climax, when the number of new infections in a single day reaches its highest point, will happen any time now. Others say that it is months away and that the virus will infect millions — or in one estimate hundreds of millions — of people first.

Public health officials want to know roughly when the peak will be — and how many will be infected — so that they can prepare hospitals and know when it will be safe to lift travel restrictions. Wuhan, the city at the centre of the epidemic, and several other nearby cities have been on lockdown since late January.

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Although peak predictions can be illuminating, some researchers warn that accuracy is difficult to achieve, especially when the data used in models are incomplete. “If you revise your predictions every week to say that the outbreak will peak in a week or two, eventually you will be correct,” says Brian Labus, who works on disease surveillance at the University of Nevada, Las Vegas.

Optimistic scenario

On 11 February, Zhong Nanshan, a prominent Chinese physician leading a panel of experts helping to control the outbreak, said that the coronavirus will possibly peak by the end of February. Zhong, who is famous for discovering the SARS virus, said the situation had improved with government control measures, such as travel restrictions and extended holidays, although he admitted that it was still a “difficult period” for Wuhan.

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So far, more than 70,000 people have been confirmed to have the disease, now known as COVID-19, in China. But many scientists assume that the number of cases is higher than is being reported. They suspect that China has too few diagnostic tests and health-care workers to confirm all cases. Some scientists also wonder whether Zhong is just trying to reassure people, given the outbreak’s impact on the economy and society.

At least one model aligns with Zhong’s estimate. Researchers at the London School of Hygiene and Tropical Medicine predict that the peak could occur anytime now. Sebastian Funk, a statistician who models infectious diseases and who coauthored the analysis, says the prediction is based on an estimate that one infected person in Wuhan was, on average, infecting between 1.5 and 4.5 others — a measure known as the virus’s effective reproduction number, or R — before the travel restrictions were introduced on 23 January. Funk estimates that at the peak around a million people, about 10% of Wuhan's population, will be infected.

Funk posted the analysis, which has not been peer-reviewed, on his institute’s website on 12 February. But he says that since it was done, a decline in the number of new cases and deaths in Wuhan suggests that infections might have already peaked. (More than 14,000 new cases were reported on 13 February, but the bump was due to authorities changing the way cases are diagnosed and not a true spike).

Worst case

Some researchers find such predictions overly optimistic. People in most Chinese cities started returning to work last week after an extended public-holiday period — opening up the possibility of new chains of transmission, says Hiroshi Nishiura, an epidemiologist at Hokkaido University in Sapporo, Japan.

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Nishiura says he has used a model that estimates that the outbreak will peak sometime between late March and late May. At this point, he says, up to 2.3 million cases will be diagnosed in a single day. In total, he estimates that between 550 million and 650 million people across China will be infected, roughly 40% of the country’s population. Nishiura says that about half of those people will show symptoms.

Nishiura says he has submitted a paper describing the model and its prediction to the preprint server medRxiv. To make such a prediction, he says that his team considered the transmission potential inherent to the new virus — the basic reproduction number known as R0, which is related to R, although it assumes that everyone in the population is susceptible to infection. The team estimates the R0 is between 1.5 and 2.

He says that his model presents a relatively simplistic outlook because it assumes that everyone in the population is susceptible. It also reflects the view that many people who have been infected are asymptomatic or not unwell enough to seek medical treatment. If that is the case, the current number of reported cases massively underestimates the number of people infected, he says.

Gabriel Leung, an epidemiologist at the University of Hong Kong, says that Nishiura’s estimates are feasible. The community has no immunity to SARS-CoV-2, the virus that causes COVID-19 so “it will sweep through”, he says.

Leung says that while those estimates sound extreme, it’s still not clear how deadly the virus is. The latest calculation of the fatality rate, in a paper published by Zhong on 9 February, suggests that there are about 1.36 deaths per hundred cases. But that number is probably too high because the authors did not consider less severe cases. (Outside China, two deaths have been reported in 500 cases.)

Quarantine measures

Leung says it's also unclear what effect, if any, that control measures, such as travel bans and quarantining people, have had on the timing and severity of the peak. Nishiura and Funk say their models excluded these measures because their efficacy is uncertain.

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Many scientists say that control measures might not, ultimately, decrease the number of people infected. But they could extend the time taken for the epidemic to peak, by slowing transmission, says Leung.

Reducing the number of people who get infected at the peak is important, says Leung. If everyone gets sick at the same time, “the entire society grinds to a halt,” he says. “Health services will be overwhelmed and people will die.”