Outbreaks of influenza are getting an early start this year in part because of cold weather gripping much of the USA and low efficacy associated with this year’s flu vaccine.
It’s still too early to say whether this winter will be a bad season for the flu, but epidemiologists in 36 states already have reported widespread influenza activity to the federal Centers for Disease Control and Prevention in data released Friday. Twenty-one of those states show a high number of cases.
“It’s just one of those years where the CDC is seeing that this strain of flu is only somewhat covered by the vaccine that was given this year,” said Jennifer Radtke, manager for infection prevention at the University of Tennessee Medical Center in Knoxville. “They’re seeing that it’s anywhere from 10% to 33% effective, so any time there’s a mismatch between the vaccine and the circulating strain of the flu, you’re going to see more cases.”
Vaccine effectiveness varies from year to year though recent studies show that a flu shot typically reduces the risk of illness by 40% to 60% among the overall population when the circulating virus is matched closely to the vaccine virus, according to the CDC.
Because only a certain percentage of people with flu symptoms go to hospitals and get tested, it can be challenging to track the actual number of people affected, Radtke said. False negative results for flu tests are also common, so it’s likely the number of people with the flu is much higher.
From the start of the flu season, which begins in October and lasts until May, Arizona has reported a nearly ninefold increase in the number of cases compared with the same period last year, according to the state Department of Health Services.
“It’s not uncommon to see (flu) this time of year,” said Radtke in Knoxville. “But we’ve had cold Decembers before and not had flu.”
Flu symptoms include fever, body aches, chills, fatigue, cough and a sore throat. The illness typically passes within a few days but can be especially dangerous to the very young, the very old, pregnant women and those with respiratory problems.
Influenza can develop into pneumonia, an infection that causes the lungs’ air sacs to become inflamed and fill with fluid.
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