• Everyone 6 months of age and older should get vaccinated against the flu as soon as the season vaccine is available.
• People at high risk of serious flu complications include young children, pregnant women, people with chronic health conditions like asthma, diabetes or heart and lung disease and people 65 years and older.

Vaccination of high risk persons is especially important to decrease their risk of severe flu illness.  Vaccination also is important for health care workers, and other people who live with or care for high risk people to keep from spreading flu to high risk people.  Children younger than 6 months are at high risk of serious flu illness, but are too young to be vaccinated. People who care for them should be vaccinated instead.

The C.D.C. identifies and tests a viral strain, then distributes a version of it to vaccine manufacturers, who inject it into millions of eggs, where it multiplies. Then the virus is harvested, purified and developed into a vaccine.

The injectable vaccine or flu shot is made from a killed influenza virus that is highly purified and broken into tiny pieces. It cannot recombine in the body to produce flu, but it can still stimulate the immune system to evoke a protective response.

The nasal vaccine, called FluMist, is made of an attenuated live virus. This is a weakened version of the virus that has been tamed in the laboratory so it cannot cause illness. (The measles and chicken pox vaccines also are made from a live attenuated virus.) The attenuated virus can multiply only in the cooler temperatures of the nasal passages, and cannot survive in the higher temperatures of the respiratory tract, said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University School of Medicine. When the vaccine is sprayed into the nose,it multiplies on the mucous membranes in the nose and throat, triggering the body’s immune response without causing any illness. Some studies suggest the nasal mist is more effective than the traditional flu shot.

FluMist is not approved for people with asthma, pregnant women or people with underlying medical problems like heart disease and diabetes.

Unless your case was officially confirmed by a laboratory test as H1N1 2009, there is no way to be sure you are protected. If you did have confirmed H1N1, you are still vulnerable to seasonal flu.

The flu vaccine is typically not recommended for patients with any signs of illness, so that symptoms are not wrongly misdiagnosed as side effects of a flu shot.

The vaccine typically is not given to people with egg allergy. People at high risk for flu complications may be able to work with an allergist to be desensitized so they tolerate the vaccine.

A flu vaccine works by stimulating the body’s immune system to produce antibodies against the virus. A person with a suppressed immune system cannot generate an immune response and does not benefit from vaccination. This includes many cancer patients undergoing chemotherapy, asthma patients who require large doses of steroids and those taking immune-suppressing drugs after an organ transplant. Patients who are immune-compromised should talk to their doctor about whether they should get a flu shot.

This is a myth perpetuated on some health websites. Although substances called adjuvants are sometimes added to vaccines to make them more effective, no flu vaccine sold in the United States, including the H1N1 2009 vaccine, contains any adjuvants.

Flu vaccine packaged in a multidose vial contains thimerosal, a preservative that prevents contamination of the vial during repeated use. One dose from a multiuse vial contains about 25 micrograms of mercury. By comparison, a tuna fish sandwich contains about 28 micrograms of mercury. Repeated studies have shown thimerosal to be safe. However, people who want to minimize mercury exposure can ask for a vaccine in a single-dose package, which has only trace amounts. Thimerosal is not used in the production of FluMist.