What is influenza? Why should I get a flu shot (influenza vaccination)? Will the flu shot make me ill?
Influenza is a serious respiratory disease caused by the influenza virus (Table 49). Influenza viruses are classified as types A, B, or C. Influenza A, the most common, is the type of influenza that causes most serious epidemics.
Influenza viruses of B type have also been responsible for epidemics of seasonal influenza illness, but tend to lead to milder illness than the A viruses. Influenza produces widespread, sporadic respiratory illness each year during fall and winter in the Northern Hemisphere. It has also been known to occur in epidemic or pandemic forms, most recently in 2009.
That year, a novel influenza A virus–the 2009 H1N1–emerged in the Western Hemi-sphere. On June 11, 2009, Dr. Margaret Chan, director-general of the World Health Organization (WHO), announced that the 2009 H1N1 virus had become a worldwide pandemic. At the time, more than 70 countries had reported cases of H1N1 infection with ongoing outbreaks of the newly identified and novel H1N1 in multiple parts of the world. WHO accordingly raised the worldwide pandemic alert to Phase 6, its highest.
H1N1 became a top priority for physicians and scientists, especially public health experts, epidemiologists, virologists, and vaccine experts. It was quickly apparent that some groups were at increased risk of complications, including death, from H1N1 infection. Those groups included children, adolescents, pregnant women, persons with asthma and with other underlying health conditions, such as obesity, diabetes, and immune deficiency.
Influenza disease is completely different from the common cold. Influenza is a specific respiratory illness, caused by one of the influenza viruses. Many people wrongly say that they “have the flu” to indicate that they are under the weather or ill in a general sort of way. The symptoms of seasonal influenza include the sudden, abrupt onset of chills and high fever in the range of 102F to 103F.
Severe, generalized aches and pains, often most intense in the back and the legs, accompany the fever. Exhaustion is common, as is headache and loss of appetite. Pain felt behind the eyeballs is often reported. Respiratory symptoms are initially mild; a scratchy or sore throat can accompany a slight dry cough. The lung symptoms develop later and come to dominate the picture with persistent and productive cough as an unmistakable feature. Fever can last up to 5 days in uncomplicated cases. After other symptoms subside, weakness and fatigue may linger for several weeks.
Unfortunately, not all persons who become ill with influenza have an uncomplicated course as just described. Severe, fulminant, or fatal pneumonia can complicate influenza. Seasonal influenza is responsible for more than 36,000 deaths and 200,000 hospitalizations each year in the United States alone.
Many of the deaths and hospitalizations occur among persons with underlying health conditions that place them at increased risk for complications. Increasing age (age 50 years or older), as well as very young age (age 2 years or younger) also place individuals at increased risk of influenza complications.
Pregnancy is a common and often underestimated risk for complications from influenza infection. Asthma is also considered a potential risk. Influenza can now be diagnosed in a few minutes in a doctor’s office. If you have symptoms of influenza, your doctor may advise you to have a nasal wash performed to be tested for the influenza virus.
Your physician will let you know if you are a candidate for immediate treatment with an antiviral medicine. The available medications include oseltamivir (Tamiflu), which is FDA approved for treatment of influenza for those aged 1 year and older, and zanamivir (Relenza), which is approved for older children and adults.
Since zanamivir is administered by inhalation, it is generally not recommended for persons with asthma, as it may cause wheezing. Both medicines shorten the time period during which you can transmit influenza to others and help to reduce the severity and duration of your illness, pro-vided they are taken very early in the course of influenza. Oseltamivir, for example, is effective only if started within 48 hours of the onset of influenza symptoms.
Taking either medicine later in the course of an established influenza infection does not provide benefit. Oseltamivir is also approved for the prevention (prophy-laxis) of influenza in exposed and at-risk adults, as well as adolescents aged 13 years and older. Neither oseltamivir nor zanamivir is considered a substitute for influenza vaccination.
The single best way to prevent influenza is vaccination. Vaccination against influenza virus causes the body’s immune system to manufacture protective antibodies. The antibodies produced in response to suc-cessful vaccination will help fight off influenza when and if a person becomes exposed to the virus. Detailed studies of the structure of the influenza virus reveal that the virus wraps itself in a protective envelope or coat.
Different strains of influenza virus carry (or express) different proteins on the surface of the viral envelope or coat. Influenza is considered a tricky virus because of its ability to change the proteins on its coat.
Even subtle changes increase influenza’s ability to invade the human body and to cause illness. Antibodies directed against one specific strain of influenza are actually specific against that particular strain’s envelope proteins.
The body thus needs to produce specific antibodies against different influenza virus envelope proteins in order to effectively protect against different strains of virus. Antibodies against one strain are unlikely to offer complete protection against a different strain that carries a very different coat or envelope.
Different strains of influenza circulate each flu season, which is why you can get the flu for 2 or more years in succession. It is also the reason that each year, government health agencies advise vaccine manufacturers on what strains to include in that particular year’s influenza vaccine. A different influenza vaccine is thus developed and produced each year. The influenza vaccine available each fall in the United States is directed against three specific influenza strains that health authorities believe will cause most of the serious illness during that year’s fall and winter influenza season.
Each year’s vaccine includes one influenza A (H3N2) virus strain, one regular seasonal influenza A (H1N1 type) virus strain (not the 2009 H1N1), and one influenza B virus strain. The vaccine pre-pared in advance of the 2009 season was thus expected to confer protection against the common disease-causing strains of influenza circulating in the United States during the winter of 2009–2010. The worldwide emergence of illness due to the novel H1N1 in the summer of 2009, after seasonal influenza vaccine production had already begun, mandated that a second vaccine be manufactured to offer protection against that strain, too.
Two types of influenza vaccines are available for preven-tion of influenza in the United States: the inactivated influenza vaccine, and the live, attenuated influenza vac-cine (LAIV) (Table 51). As of this writing, the following five inactivated influenza vaccines are approved for use in the United States: Afluria (manufactured by CSL), Fluarix, FluLaval (both manufactured by GlaxoSmith-Kline), Fluvirin (manufactured by Novartis), and Fluzone (manufactured by Sanofi Pasteur). Inactivated means that the vaccine contains a killed strain or form of influenza.
It is given by injection into a muscle. The inactivated vaccine is the familiar flu shot that has been in clinical use for many years. The LAIV contains a weakened strain of influenza. The LAIV is administered intranasally, by spraying into the nostrils. The LAIV was first licensed in the United States in 2003.
It is manufactured by Wyeth and is named FluMist. Like the inactivated influenza vaccine, LAIV stimulates the body’s production of protective antibodies directed against three strains of influenza currently in circulation. Both vaccines are administered each year, ideally in October or November. LAIV, as of this writing, is approved for administration only to healthy persons between ages 2 and 49 years. Unlike the injected inactivated vaccine, LAIV is not approved for use in pregnancy, nor is it approved for persons (of any age) with asthma.
Because influenza can be a fatal and devastating illness, especially in certain groups of individuals, vaccination is recommended for those at high risk of developing medical complications from infection with influenza. Medical complications is a term that encompasses hospitalization, severe illness manifestations, respiratory failure, and death.
All persons with asthma, including children as well as adults, are candidates for yearly influenza vaccination. Not surprisingly, persons with asthma were also candidates for the 2009 H1N1 inactivated vaccine (Table 52). Only in a few, very specific medical circumstances is vaccination absolutely contraindicated (Table 53).
When to Check With Your Doctor Before Influenza Vaccination
Consult with your physician before obtaining an influenza vaccination if:
- You have a severe allergy to chicken eggs
- You have had an allergic reaction to an influenza vaccination in the past
- You have had an adverse reaction to influenza vaccination in the past
- You have had Guillain-Barré syndrome
- You are currently ill with fever; it is better to wait until you have recovered
A person who has experienced a significant egg allergy, or an allergic reaction to a prior influenza vaccine or to any constituent of the vaccine should not receive the influenza vaccine. Anyone who has been diagnosed with a neurologic condition called Guillain-Barré syndrome should consult closely with a physician knowledgeable about the risks of vaccination. In general, influenza vaccination is very safe and effective. You cannot get the flu from influenza vaccination, nor can the vaccination cause an infection.
It is possible, however, to become ill with influenza even if you received the vaccine, as the vaccine does not protect against all strains of influenza. I remind my patients that the goal of influenza vaccination is to protect them from becoming severely ill from influenza. I still consider vaccination a success if they come down with a milder form of illness that slows them for a few days but does not lead to an exacerbation of their asthma or to hospitalization.