But other aspects of the worrisome bird flu outbreak remain mysteries, Dr. Keiji Fukuda admitted in an interview with The Canadian Press.
After a week in China meeting with officials, doctors and patients, the team of international experts sent by the WHO remains deeply worried about the potential of this new virus, which has triggered more human infections in a single month than the other well-known bird flu, H5N1, racks up most years.
For the first time since the outbreak started, China's case total did not rise Wednesday, resting at 108 cases, 23 of which have been fatal. But that good news was tempered by word that Taiwan had diagnosed a case in a man who had been working in China. This is the first infection recorded outside the Chinese mainland.
"I think it's fair to say that this avian influenza virus appears to be more infectious to people than any other avian influenza virus we know," said Fukuda of the unusual virus, which carries some genetic mutations that suggest it is better adapted to infecting mammals than regular bird flu viruses.
"(That) is different than sustaining transmission, but it is an important observation. And it's one of the reasons why we're really concerned about this virus."
Fukuda is a long-time influenza epidemiologist who worked at the U.S. Centers for Disease Control before going to the WHO. At the Geneva-based global health agency he is the assistant director general for health security and the environment.
He and the team of Chinese and international flu experts on the mission concluded that the current pattern of cases suggests there is no ongoing person-to-person spread of virus. There have been several small clusters of cases within families, but those have been seen on occasion with H5N1.
With those types of clusters, Fukuda noted, it's often impossible to tease out whether the people all contracted the virus from an animal source — infected chickens, for example — or if one person got sick and infected other people.
One of the questions for which there may be an answer relates to the odd age distribution of cases. While infections have been seen in most age groups (except teenagers), a distinct feature of this outbreak has been that older men have been the largest group infected.
In fact, a study released late Wednesday by the New England Journal of Medicine reveals that of the first 82 cases, 46 per cent were people aged 65 or older and 73 per cent of the cases were men.
That's a sharp contrast to H5N1 infections, where younger adults and children have made up a greater proportion of cases. The difference has led people to wonder whether older adults are more vulnerable to this virus for some biological reason.
"It's really a very strange age pattern," Fukuda said of the H7N9 outbreak. "But there are some explanations."
The investigations are pointing to live bird markets as the places where most infections seem to occur, he said. Meanwhile, poultry in farms around Shanghai do not appear to be infected, though that needs to be confirmed with additional testing. Most of the cases to date have been in Shanghai or surrounding provinces.
The mission was told older men in a household generally do the shopping in live bird markets. "I think the most likely explanation right now is a combination of exposure and behaviour, rather than something biological having to do with things like background immunity," said Fukuda.
"If the birds in the marketplaces are primarily where the infection is right now, then that offers a reasonable explanation for why we're seeing such an unusual age distribution. And that's very different from what we saw with H5N1, where it was really widespread in backyard flocks as well as large commercial farms."
Another puzzle had been the suggestion that few of the cases had exposure to birds, raising questions about how people were contracting an avian flu virus. Some reports out of China suggested as little as 40 per cent of infected people had had exposure to animals.
"I think some of the figures that are floating out there are simply not the right figures," Fukuda said. "In our discussions with people, both from the Shanghai CDC and the Beijing CDC, I think it's likely that the percentage of people who have had some sort of (bird) contact is higher."
The New England Journal article, by scientists from a number of Chinese health institutions, put the figure higher. Of the first 82 cases, animal exposure information was available for 77. And of those patients, 77 per cent had a history of recent exposure to chickens, ducks, pigeons, pigs and other birds and animals.
Fukuda said the mission spoke with their Chinese hosts about the importance of doing what are known as case control studies, which compare people who became sick to those who did not to try to tease out where the exposures might have taken place. He said he expects this type of study will be done.
As well, he said, Chinese scientists will be analyzing blood samples from people who were contacts of cases, poultry workers and health-care workers looking for evidence of transmission, and for infections that were mild enough not to need hospital care. But getting results from this type of study will take some time.
While live animal markets have been homed in on as ground zero for infections, it is still unclear how the virus is making its way to the markets, Fukuda said. A frustrating and dangerous facet of this virus is that though it causes severe illness in people, it is virtually invisible in birds because it doesn't make them sick.
"We don't really understand where are poultry getting infected? And what is the full range of animals that may be getting infected? And where are the birds coming into the marketplaces getting infected?"
Fukuda noted the team did not get a good handle on the distribution pattern of poultry into the markets. Another group of experts, from the Paris-based World Organization for Animal Health (known as the OIE) will soon travel to China to help further with animal-related issues, including if animal testing is being done properly.
Fukuda said based on what is known now, the move by authorities to close live bird markets in affected areas appears wise. But when asked if they should be culling birds, he said that until it's clear which animals are infected, that would be a harder sell.