Coughing, sniffling, runny noses, headaches, muscle pain and high fever.

For Elis looking to escape flu symptoms such as these, Yale University Health Services vaccinates hundreds of students at each residential college each year — but this year, immunizations may not have been as successful as expected.

The flu vaccine has proven to be much less effective this season than in previous years, with an estimated efficacy of 40-60 percent, compared with the usual 70-90 percent, said David Katz, adjunct associate professor of epidemiology and public health.

The vaccine’s low effectiveness is likely the result of a discrepancy between the flu strains prevalent this season and the strains in this year’s vaccine, according to the Centers for Disease Control and Prevention.

Here at Yale and in New Haven, the lower rate of effectiveness coincides with a higher-than-usual incidence of influenza, YUHS Director Paul Genecin said.

There are also currently an increased number of influenza cases in Connecticut, said Matthew Cartter, epidemiology program coordinator of the Connecticut Department of Public Health. According to a report released by the Connecticut DPH last week, New Haven County has experienced more cases of flu so far this year than any other Connecticut county — although this trend cannot yet be explained, since flu season is not over and current numbers are inconclusive.

Genecin said this season he has noticed more flu cases in clinics, in the hospital and in urgent care than in most years. But he said he is hesitant to directly correlate the higher occurrence of the illness with the reported decrease in vaccine effectiveness.

“It’s hard to tell when you’re just seeing a finite group of patients,” he said.

Genecin said that given the severity of this season’s flu, he is not surprised that the strains found in flu cultures this season do not match the strains present in the vaccine. He said this mismatch is probably part of the reason for the increase in flu cases at the University. Another factor, he said, could be the virulence of this year’s strain.

But Katz said there is no evidence that this year’s strain is more virulent than last year’s. He said the flu season, which usually lasts from December to March, is peaking right now, so mortality figures and assessments of the strain’s virulence have yet to be determined.

Each year, the vaccine is developed by a CDC-led group of scientists who observe the early emergence of flu strains in China, Katz said. They guess which of the active strains are most likely to become predominant and then base the vaccine’s formula on these assumptions. But sometimes the vaccine does not match the principal Western strains, during seasons such as this one, Katz said.

“It’s not a perfect guess,” Katz said. “It could be that [the current forms] are from a predominant strain in the Far East plus additional mutations, or that what looked likely to become the predominant strain stopped spreading, or a combination of the two.”

Cartter said he is not surprised by the vaccine’s lower effectiveness. The current predominant strain, not found in the vaccine, was identified in Australia last summer, which is influenza season in the Southern Hemisphere, he said.

While scientists previously knew that this strain was not being covered well by the vaccine, the fact that this strain would be particularly prevalent this year was discovered too late, after the CDC had already made its official recommendations for the vaccine’s composition.

“Public-health people expected this as a possibility for this season; it happens every few years,” Cartter said. “But overall, it’s better to have a vaccine [that has a low effectiveness] than to have no vaccine.”

The 40 to 60-percent effectiveness figure was determined from cultures taken from reported flu cases received by the CDC, Katz said. Scientists then tested the cultures for correspondence between the vaccine and the flu strain. This season, he said, only about 40 percent of the strains matched the vaccine.

While the test may not be an exact measure of absolute efficacy, Katz said, it is a good indicator of the relative performances of vaccines from different seasons.