Cold & Flu Report: Influenza Vaccination

Influenza Vaccination

How does vaccination protect you?

After vaccination, the body's immune system produces antibodies against the inactivated virus in the vaccine. If you are exposed to the real virus, the antibodies prevent the infection or reduce the likelihood of severe illness should infection occur.

The effectiveness of influenza vaccine varies, depending upon the age and immunocompetence of the vaccine recipient, and the degree of similarity between the virus strain included and the strain of circulating virus during the influenza season. With a good match, influenza vaccination has been shown to prevent laboratory-confirmed influenza illness in approximately 70% to 90% of healthy children and adults.

Under these circumstances, studies have also shown influenza vaccination to be approximately 70% effective in preventing hospitalization for pneumonia and influenza among elderly people living in the community.

Studies of elderly people residing in nursing homes have shown influenza vaccination to be 50% to 60% effective in preventing hospitalization and pneumonia and up to 85% effective in preventing death, even though the efficacy in preventing influenza illness may often be in the range of 30% to 40% among the frail elderly.

Who should receive the influenza vaccine?

Influenza vaccine is recommended for people at high risk of influenza-related complications, those capable of transmitting influenza to individuals at high risk of complications, and those who provide essential community services. Influenza vaccine is thus recommended for:

• Healthy children aged 6 to 23 months. They are at increased risk of influenza-associated hospitalization compared with healthy older children and young adults. NACI recommends their inclusion among recommended recipients of influenza vaccine.
• Healthy persons aged 2 to 64 years.
• Adults and children with chronic cardiac or pulmonary disorders (including bronchopulmonary dysplasia, cystic fibrosis and asthma) severe enough to require regular medical follow-up or hospital care.
• People of any age who are residents of nursing homes and other chronic care facilities.
• People 65 years of age or older.
• Adults and children with chronic conditions, such as diabetes mellitus and other metabolic diseases, cancer, immunodeficiency, immunosuppression (due to underlying disease and/or therapy), renal disease, anemia, and hemoglobinopathy.
• Children and adolescents (age 6 months to 18 years) with conditions treated for long periods with acetylsalicylic acid.
• People at high risk of influenza complications embarking on travel to destinations where influenza is likely to be circulating.
• People capable of transmitting influenza to those at high risk of influenza-related complications. These people include:
  — Health care providers who work in facilities and community settings, such as physicians, nurses, and emergency response workers.
  — Health care and other service providers who have contact with residents of continuing care facilities or residences.
  — Those who provide home care for persons in high-risk groups.
  — Those who provide services within closed or relatively closed settings to persons at high risk (e.g. crew on ships).
  — Household contacts (adults and children) of people at high risk of influenza complications. This includes household contacts of children < 6 months of age, who are at high risk of complications from influenza but for whom there is no currently licensed vaccine, and of children aged 6 to 23 months whether or not they have been immunized. Pregnant women should be immunized in their third trimester if they are expected to deliver during influenza season, as they will become household contacts of their newborn (unless adoption occurs).
  — Those providing regular child care to children aged 0 to 23 months, whether in or out of the home.
• People in direct contact with poultry infected with avian influenza during culling operations.
• Influenza vaccination is recommended for pregnant and breastfeeding women who are characterized by any of the above conditions. This includes pregnant and breastfeeding women who have chronic conditions that put them at high risk of complications from influenza, as well as those who are close contacts of high-risk individuals.

Who should not receive the influenza vaccine?

Current influenza vaccines are not recommended for children under 6 months of age. Influenza vaccine should not be given to people who had an anaphylactic reaction to a previous dose or with known anaphylactic reaction to eggs manifested as hives, swelling of the mouth and throat, difficulty in breathing, hypotension and shock.

Can influenza vaccination cause influenza?

No. Influenza vaccination cannot cause influenza because the vaccine does not contain living virus. All current influenza vaccines manufactured in Canada are composed of killed, split and purified virus particles, which are incapable of causing influenza.

What are the side effects of the influenza vaccine?

Soreness at the injection site lasting 2 days or less is the most common side effect of influenza vaccination, but this rarely interferes with one's normal activities. Fever, tiredness, and muscle aches may occur within 6 to 12 hours after vaccination and last 1 to 2 days. These side effects are less often seen with the "split-virus" vaccines in use in Canada.

Why is it recommended that influenza vaccine be taken every year?

The antibodies produced in response to an influenza vaccine decline over time and may be too low to offer adequate protection for the following year. In general, influenza antibodies from vaccination only lasts 4-6 months. Furthermore, influenza viruses continually undergo genetic changes; thus one's immunity, acquired as a result of previous influenza infections or previous vaccination, would not necessarily protect against subsequent infections to new strains of the virus. As the influenza viruses change, the vaccine has to be updated on a yearly basis to include the most current strains.

Is it too late to get an influenza vaccination in January?

No. Active flu season begins in November and ends in May. Getting vaccinated at anytime between these months would still be useful to protect against infection. However, you should consult with your physician or local public health authority.

Is it useful to get a vaccination if you are already coming down with the flu?

Yes. The vaccine will not help you with the current infection but it could help protect you against other influenza strains which may also be present in a given year and to which you may be susceptible.

Is it possible to become infected with influenza even if you have been vaccinated?

Yes. It is possible to become infected after vaccination, particularly in older adults whose immune systems may be less effective in responding to the vaccine. However, for those who still get infected after vaccination, the disease is likely to be less severe and less likely to result in hospitalization or death.

What factors affect the success of influenza vaccine?

• Whether an individual has experienced the types of influenza in the vaccine
• The age of the person being vaccinated
• The health of the person's immune system
• The length of time from vaccination to exposure to a flu virus
• The similarity between the virus they are exposed to and the vaccine they received

Is there anything other than vaccine that can be used to control influenza?

Amantadine is an approved antiviral medication in Canada. It can be used to protect people who have been in contact with an infected person or for individuals who may respond poorly to influenza vaccination (for example, the elderly). Amantadine is most useful in certain situations, such as outbreaks of influenza in nursing homes. It is not effective against influenza B virus. A new class of antiviral medications, the neuraminidase inhibitors, is able to inhibit both influenza A and B. The two neuraminidase inhibitors licensed in Canada for treatment of influenza are zanamivir (Relenza) and oseltamivir (Tamiflu). These drugs have been shown to reduce the duration (by an average of 1 to 1.5 days) and severity of flu symptoms when taken within 48 hours of the onset of illness. Neuraminidase inhibitors are not currently licensed for the prevention of influenza (prophylaxis). Antiviral medications should not be used to replace influenza vaccination for the prevention of influenza.

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