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Immunizations

Description

An in-depth report on the purpose of immunizations and recommended schedules.

Alternative Names

Measles; Rubella; Tetanus; Vaccinations; Whooping cough

Viral Influenza

Influenza, commonly called the flu, is always 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 the worldwide epidemics (pandemics) of the flu that have occurred. An estimated 142,000 hospitalizations per year are due to this strain of the flu. Influenza A is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Avian Influenza A (called “bird flu”) was first detected in humans in 1997 in China and the region of Hong Kong. Bird flu is spread easily from bird to bird. Humans usually contract the flu from contact with infected domesticated birds, such as chickens, turkeys, and ducks. Since 2003, the World Health Organization confirms that there were, as of the publishing of this report, 232 cases of bird flu in humans and 134 deaths. The greatest number of cases have occurred in Viet Nam (93), followed by Indonesia (54), Thailand (23), and China (19). The U.S. National Institute of Allergy and Infectious Diseases is sponsoring a clinical trial to evaluate the effectiveness of a vaccine against bird flu. However, no vaccine is available at this time.
• 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.

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. (See Flu Vaccines in this report.)

Complications of the Flu. In general, the flu is usually self-limited and not serious. It is responsible, however, for 15 - 30% of the excess number of hospitalizations that occur in winter. About 1% of people who contract the flu end up in the hospital, and an estimated 36,000 people currently die each year of flu-related complications. The highest risks for serious complications occur in people age 65 and older and in those who are already sick with another disease. There have also been reports of flu-related deaths in very young children.

Pneumonia is the major serious complication of the flu 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, flu 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.

Flu Vaccines

Description of Vaccines. Vaccines against the flu use inactivated (not live) viruses. The influenza vaccine is commonly called a "flu shot." It is 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.)

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 5 and 49 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 the flu that ranged between 66% and 92%, depending on whether the flu was A or B. (The lower rates were those observed for influenza B, particularly a new variant.) In a broad 2004 Canadian study, one to two doses of the intranasal spray offered significantly more effective protection to children (aged 15 - 71 months) than injected vaccines. However, most insurance plans do not cover the cost of the intranasal vaccine. A preservative-free intramuscular injectable 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 flu virus usually develop within 2 weeks of vaccination, and immunity peaks within 4 - 6 weeks, then gradually wanes.

• Because children under age 9 do not develop strong immune responses to one dose, the CDC recommends two vaccinations given 1 month apart.
• Early research also suggests that it may be equally effective to administer children’s vaccinations in the spring and fall, rather than 1 month apart; further study is ongoing.
• 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 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. 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, a flu shot may help protect against stroke, adverse heart events, and death from all causes.

Candidates for the Flu Vaccine

Children Who Should Be Vaccinated. The following children over 6 months should be vaccinated against the flu:

• The American Academy of Pediatrics (AAP) and the CDC recommend flu shots for all healthy children between 6 and 23 months of age. In addition, any child over the age of 2 years who has 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).
• Children who are receiving long-term aspirin therapy should also receive a flu shot. Children who get the flu are at higher risk for Reye's syndrome, a life-threatening disease.
• Some experts now advocate flu shots for all school-age children. Emerging research indicates that children are responsible for transmitting the vast majority of cases of the flu, and that routine vaccination of school-age children would considerably reduce transmission rates throughout communities.

Of note: There has been some question concerning flu shots 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 flu shots and are given top priority:

• All adults 65 years and older. Older adults who get a flu shot have lower hospitalization rates than those who do not. 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 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. (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 the flu 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.)
• All healthcare workers should be vaccinated, according to ACIP’s 2005 recommendations.
• 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 for the flu 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 the flu 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.

Side Effects

Possible side effects 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 between 2 and 24 hours after the vaccination and generally last 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-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 (one additional case per 1 million people), and does not outweigh the benefits of the vaccine.

• Review Date: 2/14/2007
• Reviewed By: A.D.A.M. Editorial: Greg Juhn, M.T.P.W., David R. Eltz, Kelli A. Stacy; previously reviewed by Harvey Simon, M.D., Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital (8/23/2006).

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