Within one to two weeks after receiving the vaccine, your body produces antibodies which will fight the virus if you’re exposed to it. Infection will be either prevented, or the severity of your symptoms will be reduced.
Although the antibodies will prevent infection with the three vaccine-included influenza strains (and several more closely related strains), there is no guarantee that you won’t get sick during winter.
The flu shot is usually given in one dose as an injection into the muscle (usually the upper arm). If children under nine have not been vaccinated before, they should receive a second vaccination one month later. Children under three should receive half the adult dose on two occasions, one month apart.
The latest vaccine – using live viruses made harmless, and administered in the form of a nasal spray – was introduced in the USA in 2008 and in Canada in 2009, but was not yet available in South Africa at the time of writing.
How effective is the influenza vaccine really?
No vaccine is 100% effective. Factors within any individual or relating to the vaccine may cause the vaccine not to take or not to provide full protection. This is even truer of flu vaccines, as scientists have to predict which strains are more likely to cause disease each year.
However, studies have shown that protection from the vaccine in healthy adults is 70%-90% when the strains are well matched, compared to 50%-80% when they are not well matched. In this group of people, the vaccine reduces hospital admissions by 90% and days off work by 43%.
These figures may change for certain groups:
• In older people in nursing homes, the vaccine confers a maximum of 40% protection against flu and reduces death from flu-related illness by 39%. These numbers marginally improve in the case of healthy senior citizens in the community.
• In children, the vaccine is reported to prevent 59%-82% of influenza cases, although the rates may be lower in children with asthma. If just a child is immunised, but not the other people in his household, the likelihood of others in the house contracting flu is reduced by 42%!
Who should be vaccinated?
Anyone who does not have a contraindication (see ‘Who should not be vaccinated?’) and who wants to reduce the likelihood of becoming ill with flu, should be vaccinated.
The immunisation recommendations of the National Advisory Group on Immunization (NAGI) of South Africa for 2010 are in order of priority:
1. Pregnant women, irrespective of stage of pregnancy
2. Those older than six months with underlying medical conditions, predisposing them to flu complications. These conditions include chronic lung disease, chronic heart disease, chronic neurological disease, chronic renal disease, mild to severe diabetes and related metabolic conditions and people on aspirin therapy.
3. Frontline healthcare and emergency medical service personnel who come into direct contact with patients
4. HIV-infected adults with a CD4 count above 100 and all HIV-infected children, six months to five years of age
5. Caregivers of infants younger than six months in day-care centres
6. Everyone older than 65
7. Children between six months and five years old
8. People between five and 24 years old who live in hostels, boarding schools and similar institutional settings.
Breastfeeding moms may also receive the flu shot without affecting the safety of the infant.
Also remember that Haemophilus influenza type B is not the cause of flu, but a bacterium that causes meningitis, ear infection, pneumonia and other infections. A vaccine against this bacterium is routinely administered to babies as part of the government’s Extended Programme on Immunisation.
Who should not be vaccinated?
Some people shouldn't be vaccinated. Examples are:
• Babies younger than six months
• Be careful if you’re allergic to eggs (the protein albumin) because the vaccine virus is grown in eggs. But if necessary, even people allergic to eggs may be vaccinated under close medical supervision.
• Be wary if you’ve experienced side effects with previous vaccinations that contained components or constituents included in the current vaccine. Under certain conditions, you may receive the vaccine in two half doses.
• Rather delay your flu shot if you’re ill with a high fever or any acute illness.
• Although there are no known adverse effects on the fetus, vaccines are generally avoided in the first trimester of pregnancy. But as pandemic influenza H1N1 is known to cause complications in pregnant women, it’s best to take the vaccination irrespective of stage of pregnancy.
• If you have a bleeding disorder, you should not be vaccinated without first consulting a healthcare professional.
Consult your doctor/healthcare professional if you think you should not be vaccinated or if you’re uncertain.
Side effects might occur in some people
The vaccine cannot cause flu, as the virus in the vaccine is dead and not infectious. Possible side effects usually consist of a low-grade fever and mild flu-like symptoms, lasting eight to 24 hours after immunisation. These symptoms are caused by the response of your immune system to the vaccine.
• Mild soreness at the site of the injection is common and lasts one to three days.
• About 1 to 9% of people may experience fever, headache, sore muscles or other symptoms resembling flu that can start within twelve hours of receiving the flu shot. It will last only 24 hours. These symptoms are not uncommon in children and the elderly.
If you start experiencing these symptoms more than a day after you’ve been vaccinated, or if they last longer than two days, it is almost certainly due to an illness unrelated to the vaccine. Consult your doctor or healthcare professional in such a case.
When is the best time to be vaccinated?
It takes the immune system about two to three weeks to produce sufficient quantities of specific antibodies against the flu strains in the vaccine. This means that the best time to be vaccinated against the flu is before the end of April, before the flu season typically starts. However, there is no cut-off date for flu vaccination; it can be given at any stage during the winter.
(Reviewed by Dr Jane Yeats, Department of Virology, University of Cape Town 2006)
(Updated and reviewed by Dr Jean Maritz and Dr Leana Maree, medical virologists, Tygerberg Hospital and University of Stellenbosch 2010)