San Francisco is having an early and nasty flu season, with test positivity rates already surpassing last winter's peak and still going up, the San Francisco Chronicle reported.
The percent of flu tests that come back positive began climbing in September, about two months earlier than usual, according to San Francisco data cited by the Chronicle. It is now 23%, higher than last season's peak of 20%. Statewide, the influenza uptick also started early and is now around 17% — lower than last season's peak of 33% but still climbing, according to state test positivity data. This update is distinct from earlier coverage of an early start to the season: the new figures show San Francisco has already cleared last winter's peak.
"Cases are high," said Dr. Brad Hare, chief of infectious diseases at Kaiser Permanente San Francisco. His guess is that the season will peak "in a few weeks," followed by a reprieve and then another uptick as a different type emerges later in the winter. "We may have a season that's up and down with flu instead of a large single peak and slow decline," he said.
It is unusual for the Bay Area and the West Coast to get hit earlier than the rest of the country. It is not clear why California is seeing this early wave, but some infectious disease experts theorize it may be related to ties and travel to Asia, which saw a wave of H1N1 over the summer — the same subtype of influenza A now circulating in the United States, the Chronicle reported. "Because of the close association of the West Coast with Asia, we saw that first in the U.S.," said UCSF infectious diseases expert Dr. Peter Chin-Hong.
H1N1 triggered the "swine flu" pandemic in 2009, and the virus that was in Japan and South Korea over the summer was a slightly mutated version of it, G155E, the Chronicle reported. An early start does not necessarily mean the season will end early. It may mean a prolonged season with a later spike of a different type, most likely influenza B. Hare said the vaccine is a decently good match for the circulating strain and that the strain does not appear to cause more severe symptoms. People who were vaccinated in September do not need a second flu shot later, Chin-Hong said. "Right now, we only recommend one flu shot for the year." Hare urged people who have put off vaccination to get a flu shot now and to watch for sore throat, cough, runny or stuffy nose, body aches and fever.