NEW YORK (AP) — The flu vaccine may not be very effective this winter, according to U.S. health officials who worry this may lead to more serious illnesses and deaths.

As flu season begins to ramp up, officials say the vaccine does not protect well against the dominant strain seen most commonly so far this year. That strain tends to cause more deaths and hospitalizations, especially in the elderly. CDC officials say the vaccine should still provide some protection, but it won't be as good as if the vaccine strain was a match.

Flu vaccine effectiveness tends to vary from year to year. Last winter, flu vaccine was about 60 percent effective overall, which experts consider good.

The Centers for Disease Control and Prevention issued an advisory to doctors about the situation Wednesday evening.

CDC officials said doctors should be on the look-out for patients who may be at higher risk for flu complications, including children younger than 2, adults 65 and older, and people with asthma, heart disease, weakened immune systems or certain other chronic conditions.

Such patients should be seen promptly, and perhaps treated immediately with antiviral medications, the CDC advised.

Among infectious diseases, flu is considered one of the nation's leading killers. On average, about 24,000 Americans die each flu season, according to the CDC.

Current flu vaccines are built to protect against three or four different kinds of flu virus, depending on the product. The ingredients are selected very early in the year, based on predictions of what strains will circulate the following winter.

In the last three weeks, flu cases has broken out in different parts of the country. Lab specimens from patients have shown that the dominant flu bug so far is a strain of H3N2 that is different from the H3N2 version in the vaccine. About 48 percent of the H3N2 samples seen so far were well matched to what's in the vaccine, but 52 percent were not, the CDC said.

This news follows another problem recently identified by CDC officials, involving the nasal spray version of flu vaccine.

At a scientific meeting at the CDC in October, vaccine experts were told of preliminary results from three studies that found AstraZeneca's FluMist nasal spray had little or no effect in children against the swine flu strain that was the most common bug making people sick last winter.

Because this year's version of FluMist is the same formulation, experts said it's possible the spray vaccine won't work for swine flu this season, either.

However, CDC officials believe H3N2 will be the most common flu bug this winter.