Add this to the list of things that were good about our warm December: It may be partly responsible for the mild beginning to this winter's flu season.

"It is a very slow season, locally, statewide, and nationally," said Caroline Johnson, director of disease control for the Philadelphia Department of Public Health. "Everything seems to be much lower than usual."

Flu is spread by droplets that infected people spray into the air when they cough or sneeze, said Loren Robinson, deputy secretary for health promotion and disease prevention for the Pennsylvania Department of Health. Transmission is most efficient when people are close together. We are more likely to huddle inside when it's frigid outside. When the weather is warm, people go out more and are not as close together.

Of course, this means that conditions have been better for spreading flu with the recent colder weather, and health departments are seeing a small uptick in confirmed cases. Flu season can last into May, but usually peaks between December and February. There is plenty of time for things to get worse. Though this weekend is expected to be a bit warmer than usual, temperatures are forecast to go down again next week.

"If the weather stays cold, I think that we're still going to see a pretty robust flu season," Robinson said.

Johnson agrees that the weather may be a factor, but said there may be more to it. "The vaccine is very effective this year," she said. In addition, one of the circulating viruses, H1N1, has been around for five years. Almost everyone except the very youngest among us has either had it by now or been vaccinated for it multiple times.

She said there is no way to know how many people have received flu shots, but vaccination rates in Philadelphia, particularly among the vulnerable elderly, were very high last flu season.

She said she was optimistic that flu will stay a relatively small problem this year. "I'm going to go out on a limb and say I don't think we're going to have very much of an active flu season, because we haven't seen any variant strains," she said, "but it could happen tomorrow."

Robinson added the advice that public health experts always give at this time of year: There's still time to get a flu shot, and it's still a good idea.

You can help your coworkers stay well by staying home if you get flu symptoms. These tend to come on suddenly, and they include several days of fever, aches, chills, respiratory symptoms, and headache. It feels a lot worse than a cold, or even the persistent cough that some people have been experiencing.

Robinson also extols the value of hand-washing.

The U.S. Centers for Disease Control flu map for the week ended Dec. 26 showed only sporadic flu activity in much of the country. The East Coast and the Southwest have more illness than other parts of the country. New Jersey had what health officials call localized flu cases, while Pennsylvania had regional activity, a higher level of cases. For the week that ended Saturday, New Jersey went up to moderate activity, and Pennsylvania was unchanged.

Johnson said many people have been coughing and sneezing this winter. They just didn't have the flu. "There's a lot of respiratory illness out there," she said, "but it's mostly the common cold."

215-854-4944 @StaceyABurling