About 100 human H7N9 bird flu cases are estimated to have gone undetected on the mainland, according to university researchers who based the projection on people's rate of exposure to poultry in markets.
Other than severe cases, there are usually mild cases of influenza in which the infected may not fall ill, said Professor Gabriel Leung, director of the University of Hong Kong Public Health Research Centre.
Of just over 100 confirmed H7N9 cases, almost all were detected when patients were admitted to hospital. Twenty have died and 12 have recovered.
Despite the undetected cases, Leung said there was only a very limited possibility of human-to-human transmission, so the risk of a large-scale pandemic was small at the moment.
"The undetected cases from our statistical deduction are not there because people concealed them, but [they are] rather undiscovered because symptoms were mild or people did not even fall ill."
Experts believe that most H7N9 patients are infected by poultry, with the risk of infection proportional to the rate of exposure to poultry. Older people are more likely to be infected.
The estimate that the number of actual human infections might be double that already known was made from a study by Leung's team of the average number of chickens bought by Guangzhou people from wet markets in 2006. The data also showed that older people bought chickens more often.
It was assumed that the statistics in 2006 were similar to those of today and that people's buying habits in affected areas were similar to that in Guangzhou.
Leung suggested serum testing to find out the actual proportion of people infected by H7N9.
In the case of an outbreak of H1N1 swine flu in 2009, there were about 23,000 reported cases in Hong Kong, but serum tests later found that 20 per cent of Hongkongers - more than a million - had been infected.
Associate professor Leo Poon Lit-man from the university's Centre of Influenza Research said the importance of finding patients with mild or no symptoms was to identify high-risk groups and shed light on prevention and treatment.
Keiji Fukuda, the World Health Organisation's assistant director for health security and environment, echoed a statement by China's health ministry that there was no evidence of person-to-person transmission.
"Right now we do not see evidence of sustained human-to-human transmission," Fukuda said in Shanghai yesterday after a three-day visit by a WHO team. But he added, "There's still a lot we don't understand" about how the virus infects people.
Xinhua reported that Shandong had identified its first suspected H7N9 infection.
Meanwhile, Hong Kong's anxiety level about H7N9 is much lower than that for Sars (severe acute respiratory syndrome) threat 10 years ago. Of more than 1,000 people interviewed from April 10 to 13, some 11 per cent thought they were likely to be infected in the next month, just half the level for swine flu and Sars.
View H7N9 map in a larger map
Click on each balloon for more information on individual patients infected: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; yellow, those who have fully recovered; and pink, those infected other types of the Influenza A virus, including H1N1.