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hort-term pain for long-term gain: While immu- nizing elderly people and vulnerable populations against infl uenza saves lives, targeting children is likely the best route to reducing influenza in the community. If the children get the fl u shot and the rest of the family do not, the attack rate of fl u drops by 40%; if parents also get the fl u shot, the attack rate drops by 80%.1Who gets infl uenza?
About 10% to 20% of people get infl uenza every winter. Th e attack rate is higher (as high as 40%) in children.
Who suff ers?
We all suffer when we get influenza. Children younger than 2 years and elderly people have more severe consequences. More than 90% of deaths among older adults can be attributed to pneumo- nia and infl uenza.2
How do we get it?
We get it primarily through infected people’s cough- ing and sneezing. Adults can be infectious from the day before symptoms begin until about 5 days after onset of illness. Children can be infectious for more than 10 days, and young children can shed virus for several days before onset of illness. Severely immu- nocompromised people can shed virus for weeks or months.
Is it infl uenza?
Evidence showing that we can discriminate infl u- enza from a common cold is limited. Symptoms that might increase the positive predictive value for infl uenza, especially in vulnerable and elderly people, are fever, cough, and acute onset (Table 13).
Test kits could be useful in guiding treatment with sialidase inhibitors, but the sensitivity of these tests
is typically only about 80% (negative results could rule out fl u).4-6
We need to give the vaccine to between nine and 13 people to prevent one adult case of infl u- enza (Table 27). Among adults older than 65, the vaccine’s effi cacy is diminished (number needed to treat: 65 to 115 to prevent one case), because their immune systems are less responsive. But the oppor- tunities for preventing more severe consequences,
Flu shot
Michael Evans, MD, CCFP
Table 1. Symptoms of infl uenza compared with symptoms of the common cold: Table might be helpful in diagnosis and is useful to show patients.
SYMPTOMS INFLUENZA COMMON COLD
Fever High, lasts 3-4 days Rare
Headache Usual, might be severe Rare
Aches and pains Usual, might be severe Mild Fatigue and weakness Usual, might be
severe, early onset
Mild
Stuff y nose Sometimes Usual
Sneezing Sometimes Usual
Sore throat Sometimes Usual
Chest discomfort, cough Usual, might be severe Mild-to- moderate Data from the Canadian Pharmacists Association.3
Table 2. Administering the fl u shot
AGE DOSE (ML) NO. OF DOSES LOCATION
6-35 mo 0.25 Two in fi rst year, 1 month apart
Only one dose annually after that
Intramuscular
<1 y: anterolateral aspect of the thigh
>1 y: deltoid injection (less limping) 3-8 y 0.5 Two in fi rst year,
1 month apart Only one dose annually after that, even if only one dose in fi rst year
Intramuscular Deltoid
>9 y 0.5 One dose Intramuscular
Deltoid Data from Health Canada.7
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such as hospitalization and death, among this group are much greater (Table 38).4
Concerns
• Soreness at the injection site for up to 2 days is com- mon, but rarely interferes with normal activities.
• Subunit virus vaccine is well tolerated. Healthy adults receiving the subunit virus had no increase in frequency of fever or other systemic symptoms compared with adults receiving placebo.
• Prophylactic acetaminophen might decrease pain at the injection site
• Th e vaccine is contraindicated for those allergic to eggs (vaccine is grown on eggs) and for those who have had a previous true allergic reaction to the fl u shot (reported about 27 times per million doses9).
Considerations
• Patients cannot get influenza from the inacti- vated fl u vaccine.
• Anyone aged 6 months or older who lives, works, or attends school in Ontario is eligible to receive the publicly funded influenza vaccine through the Universal Infl uenza Immunization Program.
• Protection lasts 4 to 6 months; the vaccine usu- ally takes about 2 weeks to work.
Vaccination can save lives
Th e following groups are at particularly high risk of infl uenza-related complications.
• Anyone with chronic heart, lung, kidney, blood, immune, or metabolic disease, including diabetes
• Anyone 6 months to 18 years old taking long- term treatment with acetylsalicylic acid
• Anyone, including a child, who lives in the same household or is in contact with people at risk, such as those mentioned above
• Seniors and anyone who lives, works, or vol- unteers in a nursing home, chronic care facility, retirement home, or other health care setting
• Children 6 to 23 months
• Health care and emergency service workers, including fi re, police, and ambulance staff
• Anyone traveling to places where the infl uenza virus is likely circulating.
Priority groups: and why
• Children and teenagers 2 to 18 years old: vaccines can reduce the attack rate of infl uenza in families by about 40% (if rest of family not vaccinated) to 80% (if family vaccinated).1
• People 19 to 65 years old: vaccines can reduce both sick days and spread to more vulnerable populations.
Systematic reviews
• Among people older than 65, the fl u shot reduced rates of hospital and outpatient visits and hos- pital admissions for influenza and pneumonia, respiratory conditions, and last (but not least) all- cause mortality.10
• Rates of influenza illness are lower (13.6% vs 22.4%) in hospitals where policy directs health care workers’ vaccination than in hospitals with no such policy.11
• A Cochrane review supported the effi cacy of child- hood infl uenza immunization but called for more high-quality trials to inform public policy.12
Best websites
• Th e best article on the topic: available at http://
www.cdc.gov/mmwr/preview/mmwrhtml/
rr5408a1.htm
• Canadian Pharmacists Association: http://www.
pharmacists.ca/content/hcp/Resource_Centre/
Practice_Resources/infl uenza_immunization.cfm
• Centre for Disease Control: http://www.cdc.gov/
fl u/
Table 3. Eff ectiveness of inactivated infl uenza vaccine: Eff ectiveness varies from year to year depending on which infl uenza virus is actually circulating in that winter.
PATIENTS POTENTIAL PREVENTION RATE
Healthy adults <65 y 70%-90% of infl uenza illness Adults >65 y 58% of infl uenza respiratory illness
30%-70% of hospitalizations for pneumonia and infl uenza
Adults >65 y in nursing homes
30%-40% of infl uenza illness 50%-60% of hospitalizations 80% of deaths
Children 1-15 y 77%-91% of infl uenza respiratory illness; no evidence of reduction in otitis media Adapted from Henley.8
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• Canadian Immunization Guide: http://www.
phac-aspc.gc.ca/publicat/cig-gci/
Common questions about the fl u shot
Why are we going after children as well? Two reasons: children younger than 2 years are more vulnerable to serious sequelae of influenza, and children and teenagers are the best targets for reducing infl uenza in the community. In Japan, the 1957 infl uenza outbreak killed about 8000 people with an attack rate of 60% and widespread school closures.13 Th is shaped public policy so that Japan became the only country to vaccinate children rather than elderly people. Th e program was eff ec- tive enough that the burden of infl uenza dropped greatly. Public support dwindled over time, and the program was withdrawn in 1994. Analysis of this change in tactics and comparing it with US data suggested the powerful eff ect on “herd immunity”
of childhood immunizations. Th e researchers esti- mate that, by immunizing children, Japan reduced winter pneumonia and influenza and prevented 37 000 to 49 000 deaths annually.13
Is the fl u shot safe and tolerable? We are becom- ing more aware that every health decision involves a risk-benefi t equation. Th e fl u shot is no diff erent.
Fortunately, because there is healthy scepticism concerning vaccines, the disadvantages of fl u shots have been well examined. Flu shots get a bad repu- tation because the perceived adverse event rate is high. Common symptoms, such as fever, tiredness, muscle aches, and headaches, are blamed unfairly on flu shots. As you can see from the placebo rate in Table 4,14 these events happened whether patients received a placebo shot or a fl u shot. Th e only signifi cant diff erence was arm soreness that resolved for most people in less than 2 days.
Th imerosal, a compound that contains minute quantities of mercury, has been used as a preserva- tive in vaccines since the 1930s and is used to reduce bacterial contamination. It has not been shown to have medical consequences. Retrospective cohort studies of large health databases have demonstrated that there is no association between childhood vac- cination with thimerosal-containing vaccines and
neurodevelopmental outcomes, including autistic- spectrum disorders.15,16 Nevertheless, in response to public concern, infl uenza vaccine manufacturers in Canada are working toward producing and mar- keting thimerosal-free infl uenza vaccines.
Infl uenza immunization is safe during pregnancy and breastfeeding. One study of infl uenza vaccina- tion of approximately 2000 pregnant women dem- onstrated no adverse fetal effects.17 Researchers estimate that one to two hospitalizations can be pre- vented for every 1000 pregnant women vaccinated.18
People who reacted to the 2000-2001 vaccine can be immunized. During the 2000-2001 season, a few people who received one of the fl u vaccines developed a generally mild side eff ect called oculo- respiratory syndrome (red eyes, respiratory symp- toms, or facial swelling occurred within 24 hours of vaccination). All people who have experienced such symptoms in the past can be safely reimmu- nized with influenza vaccine, except those who have experienced the syndrome along with severe lower respiratory symptoms (wheeze, chest tight- ness, diffi culty breathing) within 24 hours of infl u- enza vaccination. Th ese people should seek expert medical advice.
Guillain-Barré syndrome (GBS) was associated with vaccination for the 1976 swine fl u in adults. An exten- sive review of studies at the Institute of Medicine in the United States concluded that the evidence is inad- equate to accept or reject a causal relation between GBS in adults and infl uenza vaccines administered subsequent to the swine infl uenza vaccine program in 1976.19 Even if GBS were a true side eff ect of vac- cination in the years after 1976, the estimated risk of
Table 4. Adverse events of vaccinating healthy adults with fl u vaccine and with placebo
SYMPTOM FLU VACCINE (%) PLACEBO INJECTION (%)
Fever 6.2 6.3
Tiredness 18.9 19.4
Feeling “under the weather”
16 17.5
Muscle aches 6.2 5.7
Headaches 10.8 14.4
Arm soreness 63.8 24.1
Adapted from Nichol et al.14
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GBS of approximately one additional case per million people vaccinated is substantially less than the risk of severe influenza.20 Avoiding vaccinating people not at high risk of severe influenza complications and known to have experienced GBS within 6 weeks of previous influenza vaccination is prudent. In Canada, the background incidence of GBS is estimated at just over 20 cases per million people in a study from Ontario and Quebec.21
Children tolerate the flu vaccine very well. Mild local reactions, primarily soreness at the vaccina- tion site, occur in 7% of healthy children younger than 3 years old. About 12% of immunized children 1 to 5 years old have postvaccination fever.
What do I tell healthy adults about the flu shot?
The flu shot is not perfect, but definitely worthwhile.
Your chances of getting very sick from influenza and likely missing work and other responsibilities are 10% to 20%. Depending on how well the vac- cine is matched to this year’s influenza virus, get- ting the shot will reduce your risk of sickness by 70% to 90%. The only downside is that you will have a one in two or three chance of having a sore arm, but this will likely not inhibit your activities and will be gone in 2 days.
Patients cannot get the flu from the flu shot and, although some people report being under the weather after the shot, trials show this happens whether people get placebo or the real vaccine.
If you come into contact with anybody who has chronic illness, your own immunity from the shot could reduce their chances of becoming very ill.
Bottom line
• Getting the flu shot reduces severe illness in young chil- dren, elderly people, and those with chronic illnesses.
• Immunizing children is the best way to reduce the effect of influenza on families and com- munities.
• A few recipients with mildly sore arms for less than 2 days seems a small price for healthy
adults to pay when they consider that their chance of getting quite sick with influenza is 10%
to 20% each year, and the vaccine can reduce this risk by 70% to 90%.
Dr Evans is an Associate Professor in the Department of Family and Community Medicine at the University of Toronto, is Director of the Health Knowledge Laboratory at the Centre for Effective Practice, and is a staff physi- cian at the Toronto Western Hospital, University Health Network, in Ontario.
References
1. Hurwitz ES, Haber M, Chang A, Shope T, Teo S, Ginsberg M, et al. Effectiveness of influ- enza vaccination of day care children in reducing influenza-related morbidity among household contacts. JAMA 2000;284(13):1677-82.
2. Harper SA, Fukuda K, Uyeki TM, Cox NJ, Bridges CB; Advisory Committee on Immunization Practices (ACIP), Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). Morbid Mortal Wkly Rep MMWR 2005;54(RR-8):1-40. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5408a1.htm.
3. Canadian Pharmacists Association. Influenza immunization guide for pharmacists. Ottawa, Ont: Canadian Pharmacists Association; 2004.
4. Cifu A, Levinson W. Influenza. JAMA 2000;284(22):2847-9.
5. Govaert TM, Dinant GJ, Aretz K, Knottnerus JA. The predictive value of influenza symp- tomatology in elderly people. Fam Pract 1998;15:16–22.
6. Call SA, Vollenweider MA, Hornung CA, Simel D, Mckinney WP. Does this patient have influenza? JAMA 2005;293:987-97.
7. Health Canada. Canadian immunization guide. 6th ed. Ottawa, Ont: Health Canada; 2002.
8. Henley E. Prevention and treatment of influenza. J Fam Pract 2003;52(11):883-6.
9. Public Health Agency of Canada. Influenza-associated adverse events: results of passive surveillance, Canada 2001-2002. Can Commun Dis Rep 2002;28(9):69-80.
10. Vu T, Farish S, Jenkins M, Kelly H. A meta-analysis of effectiveness of influenza vaccine in persons aged 65 years and over living in the community. Vaccines 2002;20(13-14):1831-6.
11. Jordan R, Wake B, Hawker J, Boxall E, Fry-Smith A, Chen Y-F, et al. Influenza vaccination of health care workers (HCW) to reduce influenza-related outcomes in high risk patients:
a systematic review of clinical and cost-effectiveness. York, Engl: Health Technology Assessment Database; 2004.
12. Jefferson T, Smith S, Demicheli V, Harnden A, Rivetti A, Di Pietrantonj C, et al.
Assessment of the efficacy and effectiveness of influenza vaccines in healthy children: a sys- tematic review. Lancet 2005;365:773-80.
13. Reichert TA, Sugaya N, Fedson DS, Glezen WP, Simonsen L, Tashiro M. The Japanese expe- rience with vaccinating schoolchildren against influenza. N Engl J Med 2001;334:889-96.
14. Nichol KL, Lind A, Margolis KL, Murdoch M, Mcfadden R. The effectiveness of vaccina- tion against influenza in healthy working adults. N Engl J Med 1995;333:889-93.
15. Andrews N, Miller E, Grant A, Stowe J, Taylor B. Thimerosol exposure in infants and developmental disorders: a retrospective cohort study in the United Kingdom does not support a causal association. Pediatrics 2004;114(3):584-91.
16. Verstraeten T, Davis R, DeStefano F, Lieu TA, Rhodes PH, Black SB, et al. Safety of thimerosal-containing vaccines: a two- phased study of computerized health maintenance organiza- tion databases. Pediatrics 2003;112(5):1039-48.
17. Heinonen OP, Shapiro S, Monson RR, Hartz SG, Rosenberg L, Slone D. Immunization during pregnancy against poliomy- elitis and influenza in relation to childhood malignancy. Int J Epidemiol 1973;2:229–35.
18. Neuzil KM, Reed GW, Mitchel EF, Simonsen L, Griffin MR.
Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women. Am J Epidemiol 1998;148:1094–102.
19. Institute of Medicine of the National Academies.
Immunization safety review: influenza vaccines and neurologi- cal complications. Washington, DC: Institute of Medicine; 2003.
20. Lasky T, Terracciano GJ, Magder L, Koski CL, Ballesteros M, Nash D, et al. The Guillain-Barré syndrome and the 1992-1993 and 1993-1994 influenza vaccines. N Engl J Med 1998;339:1797-802.
21. McLean M, Duclos P, Jacob P, Humphries P. Incidence of Guillain-Barré syndrome in Ontario and Quebec, 1983-1989, using hospital service databases. Epidemiology 1994;5:443-8.
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