Immunizations

Description

Alternative Names

Viral Influenza

Influenza, commonly called the flu, is always caused by a virus. Viruses are basically gene packages wrapped in protein membranes and coated with a fatty envelope spiked with molecules called glycoproteins. Three strains of influenza have been identified according to the make-up of the glycoprotein spikes and whether the viruses have one or two membranes. The two major influenza strains referred to as A and B:

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

• Influenza A is the most widespread and can even animals and humans. Influenza A is the cause of the major pandemics (worldwide epidemics) of influenza that have occurred. It is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Influenza B infects only humans. It is less common than Type A, but is often associated with specific outbreaks, such as in nursing homes.

Based on a final analysis of the 2001-2002 flu season, nearly 90% were type A and about 10% were type B. Influenza A usually causes more severe disease than type B. However, because influenza B has been less common in the past few years, there is some concern that some people--particularly small children--may have fewer antibodies to it and so may be at higher risk for severe infection.

Complications of Influenza. In general, the flu is usually self-limited and not serious. Influenza is responsible, however, for 15% to 30% of the excess number of hospitalizations that occur in winter. About 1% of people who contract the flu end up in the hospital. An estimated 36,000 people currently die each year of influenza-related complications. The highest risks for serious complications occur in people over 65 years old and in those with other medical conditions. There have also been reports of influenza-related deaths in very young children. Influenza A is the most severe strain and causes an estimated average of 142,000 hospitalizations per year. Influenzas B and C tend to be milder.

Pneumonia is the major serious complication of influenza and can be very serious. It can develop about five 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.

Note on Pandemics. Every year, influenza strikes millions of people worldwide. Influenza epidemics are most serious when they involve a new strain against which most people are not immune. Such so-called pandemics can infect more than one fourth of the world's population within a three-month period. For example, the Spanish flu in 1918 and 1919 killed 20 million people in the US and Europe and 17 million in India. Although pandemics are still of great concern, there have been major improvements in private and public health since then, including the discovery of antibiotics to treat bacterial complications, new anti-viral agents and vaccines, and intensive world-wide surveillance of outbreaks.

Viral Influenza Vaccines

Description of Vaccines. Vaccines against influenza employ inactivated (not live) viruses. They are designed to provoke the immune system to attack antigens contained on the surface of the virus. (Antigens are foreign molecules that the immune system specifically recognizes as alien and so targets for attack.)

Click the icon to see an image of antigens.

Unfortunately, the antigens in these 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 then redesigned annually to match the current strain.

• Influenza A. The influenza A virus is further categorized by primary molecular antigens (hemagglutinin and neuraminidase), which serve as the targets for the vaccines. Influenza A is a particular problem because it can infect other species, such as pigs or chickens, and undergo major genetic reassortments.
• Influenza B viruses tend to be more stable than influenza A viruses, but they too vary. Although influenza B has been far less common than A, a vaccine for type B is important because experts are concerned that small children will not have developed any immunity to the virus and will experience severe flu if they are exposed to type B.

Until recently the vaccine has been administered only with injection. A vaccine (FluMist) that can be delivered in a nasal spray has now been approved for people between five and 50 years of age. 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. Studies in 2003 reported protection against influenza that ranged between 66% and 92%, depending on whether influenza was A or B. (The lower rates were those observed for influenza B, particularly a new variant.) A preservative-free vaccine (Fluzone) is also now available.

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 influenza virus usually develop within two weeks of vaccination, and immunity peaks within four to six weeks, then gradually wanes.

In healthy adults, immunization typically reduces the chance of influenza by about 70% to 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. The major outstanding question is whether the vaccination prevents complications of serious illness. One 2003 study, for instance, reported no reduction in severity of chronic lung diseases among vaccinated patients with asthma, emphysema, or chronic bronchitis. Some evidence suggests, on the other hand, that among the elderly, influenza vaccination may help protect against stroke, adverse heart events, and death from all causes.

Candidates for the Influenza Vaccine

Children Who Should Be Vaccinated. The following children over six months should be vaccinated against influenza:

• Any child with a condition that requires regular medical care or who has been hospitalized for a serious illness (particularly lung or kidney disease, diabetes, sickle-cell, or immune deficiencies). Vaccinations even in infants under six months old with certain conditions are warranted. In fact, the American Academy of Pediatrics (AAP) and the CDC recommend the vaccination in all healthy children between six months and two years of age. This recommendation may vary from year to year depending on the supply of the vaccine.
• Children who are receiving long-term aspirin therapy should also be immunized against the flu because they are at higher risk for Reyes syndrome, a life-threatening disease, if they get the flu.

Of note: There has been some question concerning influenza vaccinations 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. Still, 90% of asthma patients remain unvaccinated.

Older Children and Adults Who Should Be Vaccinated. The following in order of priority are the population groups who should be vaccinated each year. The first two groups have the highest need for influenza vaccinations and are given top priority:

• All adults 65 years and older. Vaccinated older adults have lower hospitalization rates than unvaccinated peers. Evidence now suggests that vaccination may help protect against adverse heart events (including after heart surgeries), stroke, and death from all causes in the elderly. Still, studies suggest that only two thirds of 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 influenza. Such people include those with heart disease, lung problems, immune deficiencies, diabetes, kidney disease, or chronic blood disease. (There have been concerns about the safety of the vaccinations in certain high-risk patients such as those with HIV or asthma. Studies now suggest that the vaccine is generally safe in these patient groups. Furthermore, their risk for serious complications from influenza outweighs any potential adverse effects from the vaccines.)
• Adults between the ages of 50 and 64 who have chronic medical conditions. (The US Advisory Committee on Immunization Practices (ACIP) suggests that all adults over age 50 should be vaccinated, although this is not recommendation of the CDC.) People (such as household members or healthcare workers) in contact with individuals who are at high-risk for complications from influenza.

Other adults who should consider influenza vaccinations include the following:

• People at risk for complications for influenza and 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 influenza and who will be in their second or third trimester during flu season. (Vaccinations should usually be given after the first trimester. Exceptions may be women who are in their first trimester during flu season and 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.

Negative Effects

Possible negative responses include the following:

• 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 one or two 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 between two and 24 hours after the vaccination and generally last up to two days. It should be noted that these symptoms are not influenza 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-Barre Syndrome. Isolated cases of a paralytic illness known as Guillain-Barre syndrome have occurred, but if there is any higher risk, it is very small (an additional one case per one million people), and does not outweigh the benefits of the vaccine.