An egg shortage has driven up prices at the supermarket, caused in part by an outbreak of avian flu in the Midwest. But health officials say there is no need to worry about the supply of certain eggs outside the kitchen: the ones used to grow and incubate flu vaccine for humans.

Vaccine manufacturers get their eggs from chickens raised under heightened sanitary and biosafety measures not in place in the typical shed.

"It's a totally separate supply," said Lynnette Brammer, an epidemiologist with the federal Centers for Disease Control and Prevention.

What's more, production for the coming flu season got underway last winter and spring, as usual, and drug makers started shipping vaccines to pharmacies and physicians in midsummer.

Sanofi Pasteur, with U.S. headquarters in Swiftwater, Pa., says it plans to ship more than 65 million doses to the U.S. market this flu season. GSK, whose commercial U.S. vaccine operations are based in Philadelphia, says it expects to ship from 32 million to 38 million doses.

And while it is too early to be certain, evidence suggests that the vaccines could be a good match with the strains of flu that are expected to dominate, health experts say. That would be a switch from last year, when the vaccine was just 19 percent effective in preventing influenza-related medical visits, the CDC said.

In the Southern Hemisphere this year, where winter is now drawing to a close, reports indicate a relatively mild flu season, said Neil Fishman, infectious diseases physician at the Hospital of the University of Pennsylvania.

"That usually predicts a really mild influenza season in the Northern Hemisphere," he said.

Flu vaccines come in various forms, and two words patients may hear at the doctor's office increasingly are trivalent and quadrivalent.

Trivalent flu shots are designed to protect against two strains of influenza A and one of influenza B. Quadrivalent vaccines are meant to protect again two of each.

Federal officials have not recommended one over the other, though GSK, which sells the quadrivalent variety, says more variety offers broader protection. Sanofi also offers quadrivalent vaccines, as well as a high-dose trivalent version for seniors, because the body's immune defenses tend to weaken with age.

This year, the trivalent-vs.-quadrivalent question may not matter, said Penn's Fishman. Most of the Southern Hemisphere cases of influenza B appear to match the strain used to make the trivalent vaccine, he said.

But it is too soon for a firm prediction about which strains will dominate here, said David C. Damsker, director of the Bucks County Health Department. A smattering of flu cases have occurred in the United States this summer, as usual, but reading too much into those is unwise, he said.

"The predominant strain doesn't always show itself in the July-August time frame," Damsker said. "You can't really make a case one way or the other just yet."

One thing physicians recommend overwhelmingly is to be vaccinated, as each year the flu kills thousands of seniors and others with weak immune systems.

The CDC recommends a yearly flu vaccine for everyone 6 months of age and older, if possible by October, as it takes about two weeks after getting a shot for peak immunity to build.

Though the flu shot does not provide 100 percent protection, doctors say it is the best thing available.

Most people who get flu will recover on their own. But seniors and others who are more vulnerable should seek medical treatment.

A new CDC study suggests that prompt treatment with antiviral medicine can shorten the time seniors with severe flu need to be hospitalized, and also reduces the chance that they will need long-term care after being discharged.

215-854-2430 @TomAvril

This story has been corrected. An earlier version misspelled the name of the Bucks County Health Department director, David C. Damsker.