TOKYO -- As the number of confirmed coronavirus cases neared 25,000 Wednesday, experts warn that this may just be the tip of the iceberg, with some drawing parallels to the fast-spreading, long-lasting swine flu outbreak of a decade ago.
Several studies suggest that the real number of cases is much higher than official reports indicate. Hiroshi Nishiura, a professor at Japan's Hokkaido University, said Tuesday that more than 100,000 people likely have been infected with the virus, and raised the possibility that nearly half caught it from carriers showing no symptoms.
Nishiura predicted that the epidemic will peak around April and stressed the need for preparations for patients at risk of becoming severely ill, in case the outbreak turns into a pandemic.
A study in late January by U.K.-based Lancaster University went further, forecasting that roughly 200,000 people in Wuhan, China, alone -- the epicenter of the outbreak -- would be infected by Tuesday. The University of Hong Kong released a report estimating the number of Wuhan cases at more than 75,000 as of Jan. 25.
Koichi Otsuki, a professor emeritus at Japan's Tottori University, sees the outbreak progressing similarly to the H1N1 swine flu outbreak of 2009 and 2010.
The new virus, 2019-nCoV, has so far proven less deadly than the severe acute respiratory syndrome outbreak of 2002 and 2003, which was also caused by a coronavirus. Official data puts the fatality rate at around 2%, compared with 10% for SARS.
The actual mortality rate is likely to be even lower, as "it's very likely that there are many cases that governments haven't detected," said Otsuki, who sees the real figure at 0.3% to 0.6%.
This is on par with H1N1, which had a mortality rate of less than 0.5%. H1N1 and the novel coronavirus also share similar symptoms, such as fever.
The World Health Organization estimates the basic reproduction number of the new virus -- the average number of cases caused by each infected person -- at between 1.4 and 2.5.
"Its level of transmissibility should stay about the same going forward," said Tetsuya Mizutani, a professor at the Tokyo University of Agriculture and Technology, noting a lack of confirmed "superspreaders" that can infect large numbers of people.
More lethal illnesses tend to spread less widely, as they often kill hosts before they can infect others. The WHO declared the SARS outbreak to be contained eight months after the first patient was discovered in China's Guangdong Province in November 2002.
But a less deadly pathogen like the H1N1 flu virus can continue to propagate and spread to new carriers, letting it persist in the population for longer. The new coronavirus is probably the latter type, Mizutani said.
Other experts are reluctant to draw conclusions at this early stage. Some argue that another week and a half will be needed to get a clear picture of the virus's infectiousness and lethality.
"This is a large-scale outbreak, and we haven't fully determined its characteristics," said Hitoshi Oshitani, a professor of virology at Tohoku University School of Medicine.
"We need to pay attention to how it peaks and fades in Wuhan and other Chinese cities," he said.
After the initial H1N1 outbreak, the virus stuck around as a new type of seasonal flu -- one of this year's most prevalent strains.
Kazuyoshi Ikuta, head of the microbiology department at the Osaka Institute of Public Health, said the new coronavirus could become another cause of the common cold. Four coronaviruses are already endemic in the human population, accounting for one-fourth of all colds, according to American medical news site Stat.
Wuhan was placed under lockdown on Jan. 23, and the Chinese government imposed a ban on tour groups from China on Jan. 27. Any infections caused by the last groups to enter Japan before the ban would appear after an incubation period estimated at a maximum of 10 days.
"If no new patients emerge in Japan by next week, we should be able to consider the danger of a major outbreak to be lessened," said Mitsuyoshi Urashima, a professor of molecular epidemiology at Jikei University School of Medicine in Tokyo. The total number of Japanese cases reached 35 Wednesday after 10 people on a cruise ship tested positive for the virus.
Koji Wada, a professor at the International University of Health and Welfare who researched flu pandemics for the Japanese government, stressed the need for hospitals to be ready.
Medical institutions "should discuss ahead of time how they will work together to respond if the outbreak spreads," he said.