While the deadliest flu season in Washington state in at least five years is winding down, more cases and deaths are likely, state health officials say.

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The deadliest flu season in Washington state in at least five years is winding down, but it’s not yet over.

State health officials say state labs have confirmed at least 120 flu deaths since the season started in September, but only a fraction of those who die from the flu are tested for the virus.

“We know the total is much higher than that,” said Donn Moyer, spokesman for the Washington Department of Health.

The deaths are underreported because influenza may not be listed as a cause, testing may not have been performed and some test results may not have been reported to public-health officials.

Last year’s full season left 79 people dead, according to lab reports. Confirmed deaths from the previous three seasons totaled 54, 18 and 36.

The state did not track flu deaths in the same way before 2010, but 60 confirmed deaths would be a more typical number, Moyer said.

The flu was at its highest level in late January-early February in Washington state. It peaked nationally on New Year’s Day.

The state does not track how many people get the flu, because it is so common. Moyer said it’s not clear whether more people died because this was an especially widespread flu season or because the strain was especially virulent.

“Flu season appears to have peaked, though there’s still considerable flu out there,” Moyer said. There are still people hospitalized with the flu in Washington state and more deaths are likely, he added.

Students at Washington State University kicked off the flu season with a rush of diagnosed cases in late September, but flu cases since the new year have been lower than expected on campus, according to Dr. Dennis Garcia, medical director at the WSU Health and Wellness Services.

Garcia credited an increase in student vaccinations — 300 more than usual — for cutting the season short.

Officials with the Centers for Disease Control and Prevention predicted this would be a tough flu season and they were correct, in part because the flu vaccine wasn’t working very well against the strain that made the most people sick this season.

Overall the flu vaccine was only 23 percent effective this year, according to the CDC. Flu vaccines are 50 to 60 percent effective in their most useful seasons.

The most common strain this year was a mutation of the H3N2 strain. Seasons when H3N2 viruses are most common tend to be the most severe, with higher numbers of hospitalizations and deaths, according to Michelle Harper, health educator with the Washington Department of Health.

Most of the deaths have been in older people with underlying health problems, but one previously healthy child has died in Washington this flu season, as well as two healthy people in the 30- to 40-year-old range.

At least 26 deaths have been reported in King County. But the county likely saw closer to 200 flu deaths, including unreported cases, estimated Dr. Jeff Duchin, interim health officer for Public Health-Seattle & King County.

Duchin called it a moderately severe season but added that it’s the most deadly in the past five years, with more outbreaks than usual in long-term-care facilities.

“Influenza is still out there, but we’re clearly past the peak,” Duchin said.

Duchin, who was at the CDC in Atlanta this past week, said next year’s vaccine will likely be more effective since it will include the mutated strain that was not well covered this year and strains of the vaccine usually hang around for a few years.

However, he said, “There is a bit of unpredictability in the influenza business.”