Cochrane for Clinicians
Putting Evidence into Practice
Vaccines for Preventing Influenza in Healthy Children, Healthy Adults, and Older Adults
Am Fam Physician. 2019 Aug 1;100(3):143-146.
Author disclosure: No relevant financial affiliations.
Do influenza vaccines reduce the risk of influenza in healthy children, healthy adults, and older adults?
Influenza vaccination reduces rates of laboratory-confirmed influenza and symptomatic influenza-like illness in healthy children, healthy adults, and older adults. There is no consistent evidence that influenza vaccination reduces school absenteeism in children, parental absenteeism from work, or adult hospitalizations, nor is there conclusive evidence that influenza vaccination decreases mortality.1–3 (Strength of Recommendation: A, based on consistent, good-quality patient-oriented evidence.)
Most people infected with influenza or viruses causing influenza-like illness (i.e., conditions that produce similar symptoms, including fever, headache, muscle aches, cough, and rhinorrhea) recover without long-term sequelae. However, serious illness and death can occur, especially among young children, pregnant women, those with chronic medical conditions, and older adults.4 Less than 25% of patients with influenza-like illness test positive for confirmed infection with viral influenza,5 so much of the morbidity attributed to influenza may be caused by other viruses. The Cochrane Library simultaneously published three reviews that stratified patients into cohorts of children, adults, and older adults.1–3 The authors of these reviews sought to determine if vaccination against influenza decreases the incidence of influenza, influenza-like illness, or related complications.
The review of children included 41 randomized controlled trials (RCTs) with more than 200,000 patients between two and 16 years of age, plus 12 case-control and 21 cohort studies. The review compared either live attenuated influenza vaccine (LAIV) or inactivated influenza vaccine with placebo.1 Eighteen of the RCTs addressed vaccine effectiveness, whereas the remainder addressed vaccine safety only. Studies were conducted in the United States, Western Europe, Russia, and Bangladesh between 1984 and 2013, mostly over single influenza seasons. Although individual comparisons of LAIV or inactivated influenza vaccine against placebo in various age groups led to different estimates of absolute and relative effectiveness, the small number of studies used for each comparison precluded definitive age-stratified conclusions on vaccine effectiveness. The reviewers instead reported overall effectiveness estimates for LAIV and inactivated influenza vaccine stratified in relation to estimated baseline rates for low-, moderate-, and high-risk groups because of wide variation in event rates across the included studies.
The reviewers found that in a moderate-risk population of children, vaccination of seven children with LAIV or five children with inactivated influenza vaccine would prevent one case of laboratory-confirmed influenza in a season. However, in the same population, vaccination of 19 children with LAIV or 13 children with inactivated influenza vaccine would be necessary to prevent one episode of influenza-like illness. The reviewers found inconsistent evidence suggesting that the incidence of otitis media might be decreased by LAIV but increased by inactivated influenza vaccine. The estimates of effect size for prevention of otitis media were too broad to allow any firm conclusion of risk or benefit. Similarly, the estimates of effect size for reducing the number of days children missed school (LAIV and inactivated influenza vaccine) were too broad to provide any firm conclusions regarding risk or benefit. No information in the reviews addressed the impact of LAIV or inactivated influenza vaccine on influenza-related deaths in children. The results suggested LAIV might be more effective with a two-dose schedule compared with a one-dose schedule. All studies of inactivated influenza vaccine used single-dose schedules. Approximately one-half of the RCTs addressing vaccine efficacy or effectiveness were at low risk of bias, whereas the remainder were at high or uncertain risk of bias related to randomization, allocation, blinding, or missing data.
The review of adult vaccination studies identified 52 clinical trials of more than 80,000 healthy people 16 to 65 years of age and reported data from 25 RCTs comparing inactivated parenteral influenza vaccine and placebo.2 Studies were conducted in North America, South America, and Europe between 1969 and 2009, mostly over single influenza seasons. The reviewers found that vaccination of 71 adults would prevent one case of laboratory-confirmed influenza in a season. Because baseline risk of influenza-like illness varied significantly across studies, from 0.6% (low) to 3.4% (moderate) to 14.6% (high), the authors combined estimates of absolute and relative effectiveness and reported summary estimates in relation to all three levels of baseline risk. For preventing influenza-like illness, the reviewers found that in a moderate-risk population, vaccination
Referencesshow all references
1. Jefferson T, Rivetti A, Di Pietrantonj C, et al. Vaccines for preventing influenza in healthy children. Cochrane Database Syst Rev. 2018;(2):CD004879....
2. Demicheli V, Jefferson T, Ferroni E, et al. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev. 2018;(2):CD001269.
3. Demicheli V, Jefferson T, Di Pietrantonj C, et al. Vaccines for preventing influenza in the elderly. Cochrane Database Syst Rev. 2018;(2):CD004876.
4. Grohskopf LA, Sokolow LZ, Broder KR, et al. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices—United States, 2018–19 influenza season. MMWR Recomm Rep. 2018;67(RR-3):1–20.
5. Thomas RE. Is influenza-like illness a useful concept and an appropriate test of influenza vaccine effectiveness? Vaccine. 2014;32(19):2143–2149.
6. Jefferson T, Rivetti A, Demicheli V. Why have three long-running Cochrane reviews on influenza vaccines been stabilised? Accessed September 17, 2018. http://community.cochrane.org/news/why-have-three-long-running-cochrane-reviews-influenza-vaccines-been-stabilised
7. Centers for Disease Control and Prevention. Key facts about seasonal flu vaccine. Accessed September 17, 2018. https://www.cdc.gov/flu/protect/keyfacts.htm
These are summaries of reviews from the Cochrane Library.
This series is coordinated by Corey D. Fogleman, MD, Assistant Medical Editor.
A collection of Cochrane for Clinicians published in AFP is available at https://www.aafp.org/afp/cochrane.
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