Viral Influenza:

Influenza, commonly called the flu, is always caused by a virus.

Influenza, also known as the flu, is caused by a virus.

There are different strains of influenza:

• Influenza A is the most widespread and most severe strain. It can affect both animals and humans. Influenza A is the cause of all the worldwide epidemics (pandemics) of the flu that have occurred. More than 200,000 hospitalizations per year are due to this strain of the flu.
• Influenza B infects only humans. It is less common than type A, but is often associated with specific outbreaks, such as in nursing homes. Flu caused by this strain tends to be milder than that caused by Influenza A.

Complications of the Flu. Pneumonia is the major serious complication of the flu and can be very serious. It can develop about 5 days after viral influenza. It is an uncommon event, however. It nearly always occurs in high-risk individuals, such as the very young or very old, and hospitalized or immunocompromised patients.

[For more information, see In-Depth Report #94: Colds and the flu.]

Flu Vaccines

Description of Vaccines. The influenza vaccine is designed to provoke the immune system to attack antigens on the surface of the virus. (Antigens are molecules that the immune system recognizes as alien and targets for attack.)

Unfortunately, the antigens in influenza viruses undergo genetic alterations (called antigenic drift ) over time, so they are likely to become resistant to a vaccine that worked in the previous year. Vaccines are redesigned annually to match the current strain.

Until recently the vaccine has been administered only by injection. A vaccine (FluMist) that can be delivered in a nasal spray has now been approved for healthy people aged 2 - 49, who are not pregnant. The vaccine contains live viruses that have been engineered to replicate in the cool temperatures of the nasal passages, but not in the warmer lungs and lower airways. Its presence in the nasal passages boosts the specific immune factors in the mucous membranes that fight off the epidemic viruses. A preservative-free intramuscular injectable vaccine (Fluzone) is also now available.

The avian flu vaccine is designed for people aged 18 - 64 who are at risk for exposure to the avian H5N1 virus. The vaccine is given as 2 doses, spaced about 1 month apart. In studies, the vaccine appeared to be effective and well tolerated. Currently, the government is stockpiling the vaccination in case of an avian influenza outbreak. The vaccine is not available to the general public.

Timing and Effectiveness of the Vaccine

Ideally, appropriate candidates should be vaccinated every October or November. However, it may take longer for a full supply of the vaccine to reach certain locations. In such cases, the high-risk groups should be served first.

Antibodies to the flu virus usually develop within 2 weeks of vaccination, and immunity peaks within 4 - 6 weeks, then gradually wanes.

• Children younger than 9 years of age, who have not been previously vaccinated or received only 1 dose the previous year should be given 2 vaccine doses, spaced 4 weeks apart.
• It should be noted that if an individual develops flu symptoms and is accurately diagnosed in time, vaccination of the other members of the household within 36 - 48 hours affords effective protection to those individuals, according to a 2004 Canadian analysis of multiple studies.

In healthy adults, immunization typically reduces the chance of getting the flu by about 70 - 90%. The current flu vaccines may be slightly less effective in certain patients, such as the elderly and those with certain chronic diseases. Some evidence suggests, however, that even in people with a weaker response, the vaccine is usually protective against serious flu complications, particularly pneumonia. Some evidence suggests that among the elderly, a flu shot may help protect against stroke, adverse heart events, and death from all causes.

Candidates for the Flu Vaccine

The American Academy of Pediatrics (AAP) and the CDC recommend flu vaccinations for all healthy children ages 6 months to 18 years.

There has been some question concerning flu vaccines because of some reports that vaccines may worsen asthma. Recent and major studies have been reporting, however, that the vaccination is safe for children with asthma. It is also very important for these patients to reduce their risk for respiratory diseases.

The following, in order of priority, are the adult population groups who should be vaccinated each year. Any person not mentioned in these categories below is still encouraged to get the vaccine if they wish to avoid the flu. The first 2 groups below have the highest need for flu vaccinations and are given top priority:

• All adults 65 years and older. Older adults who get a flu vaccine have lower hospitalization rates than those who do not. This vaccine, may also help protect against adverse heart events (including those following heart surgeries), stroke, and death from all causes in the elderly. Still, studies suggest that only two-thirds of people in this group are vaccinated, mostly because of unwarranted fears of ineffectiveness or adverse effects.
• People of any age at high risk for serious complications from the flu. Such people include those with heart disease, lung problems, immune deficiencies, diabetes, kidney disease, or chronic blood disease. Those with any condition that may compromise respiratory function or the handling of respiratory secretions, including people with cognitive dysfunction, spinal cord injuries, seizure disorders, or other neuromuscular disorders, are included in this group.
• Adults aged 50 - 64 who have chronic medical conditions. The U.S. Advisory Committee on Immunization Practices (ACIP) suggests that all adults over age 50 should be vaccinated.
• All health care workers should be vaccinated.
• Household members in contact with individuals who are at high risk for complications from the flu should be vaccinated.

Other adults who should consider flu shots include:

• People at risk for complications of the flu who are traveling to the tropics at any time or to the Southern Hemisphere between April and September.
• Pregnant women who are at risk for complications of the flu and who will be in their second or third trimester during flu season. Women who are pregnant should receive only the inactivated flu vaccine. (Vaccinations should usually be given after the first trimester. Exceptions may be women who are in their first trimester during flu season, because their risk from complications of the flu is higher than any theoretical risk to the baby from the vaccine.)
• People such as firemen or policemen who are critical for public safety.
• All others should also consider flu vaccination if vaccine supply is sufficient.

Side Effects

Possible side effects of the flu vaccine include:

• Allergic Reaction. Newer vaccines contain very little egg protein, but an allergic reaction still may occur in people with strong allergies to eggs.
• Soreness at the Injection Site. Up to two-thirds of people who receive the influenza vaccine develop redness or soreness at the injection site for 1 or 2 days afterward.
• Flu-like Symptoms. Some people actually experience flu-like symptoms, called oculo-respiratory syndrome, which include conjunctivitis, cough, wheeze, tightness in the chest, sore throat, or a combination. Such symptoms tend to occur 2 - 24 hours after the vaccination and generally last for up to 2 days. It should be noted that these symptoms are not the flu itself but an immune response to the virus proteins in the vaccine. (Anyone with a fever at the time the vaccination is scheduled, however, should wait to be immunized until the ailment has subsided.)
• Guillain-Barré Syndrome. Isolated cases of a paralytic illness known as Guillain-Barré syndrome have occurred, but if there is any higher risk following the flu vaccine, it is very small (one additional case per 1 million people), and does not outweigh the benefits of the vaccine.