DES MOINES — Flu shot procrastinators take warning, your window of opportunity is closing.

Dr. Patricia Quinlisk, medical director at the Iowa Department of Public Health, said flu season usually takes off after the Thanksgiving holiday, when people travel and gather in groups with friends and families.

Flu-shot vaccines take from one to two weeks to become effective, so the clock is ticking for Iowans as the high-risk period for contracting one of the three strains of influenza — H1N1 (2009 pandemic strain), H3N2 (regular seasonal flu strain) and Influenza B — which already have been detected in the state, she said.

“At Thanksgiving, people get into groups and do a lot of traveling and are in airplanes or cars, and we know all of those are risk factors for being exposed to people with the flu,” Quinlisk said. “Right now would be the time for everybody to make sure that they get the vaccine so that they’re protected for those holidays and all the travel.”

Flu activity in Iowa remains sporadic, Quinlisk told members of the state Board of Health on Wednesday, but she said the identification of three circulating flu strains means that an individual could become ill with the flu three different times with a vaccination that covers all three types.

“This is a little bit unusual to have all three strains show up this early, and it sort of means that we could have a very bad year,” she said, noting that flu season usually peaks in Iowa in mid- to late January.

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She said more than 350,000 doses of seasonal influenza vaccine had been administered through October, which was more than last year, but 300,000 Iowans on average get the flu each year and — together with complications from pneumonia — the serious respiratory illness ranks among the state’s top 10 causes of death.

State health experts and the federal Centers for Disease Control and Prevention recommend annual influenza vaccinations for everyone 6 months of age and older, she said.

Also, Quinlisk told the state board that Iowa essentially has ended the 2013 season for West Nile Virus with 42 human cases reported, which was up from 31 reported cases in 2012.

Iowa also has had three of the 11 cases reported nationally of infant botulism, a potentially life-threatening illness that can occur when an infant ingests bacteria that produces a toxin inside the body, she said. This year also has seen an increase in cases of “walking pneumonia,” primarily in children and young adults in Iowa, she said.