One sign would be a spate of cases among people who have had no apparent contact with poultry or environments contaminated by the feces, uncooked blood or other fluids of poultry. The other would be a series of cases in which several members of the same family fall ill in quick succession and appear to have transmitted the disease to one another.

Helen Yu, a spokeswoman for the Beijing office of the World Health Organization, wrote in an email that the proportion of cases among people who had no contact with poultry had stayed low since the disease emerged nearly a year ago and showed no sign of increasing this winter.

Similarly, there has been only one family cluster of cases this winter, compared with four clusters last spring.

“It is possible that limited human-to-human transmission may occur, but there is no evidence of sustained or widespread human-to-human transmission,” she wrote. “We continue to expect sporadic human cases.”

Extensive testing for bird flu may also result in more cases being detected even if the actual rate of infections is not increasing as rapidly as the data on confirmed infections might suggest.

Yet the H7N9 virus remains a particular concern for two reasons.

It has a series of genetic mutations that have been associated in other viruses with greater adaptation to human-to-human transmission. And the H7N9 virus has proved itself to be “much, much better than other avian influenza viruses” at growing in human lung tissue samples in a laboratory at Hong Kong University, said Dr. Malik Peiris, a prominent avian influenza researcher at the university.