Clinical question
How effective are oseltamivir and zanamivir for decreasing postexposure transmission of influenza?
Bottom line
For institutionalized seniors, 6 weeks of oseltamivir or 14 days of zanamivir will prevent 1 additional influenza case in every 25 to 27 patients treated. For every 7 to 8 households given postexposure prophylaxis (PEP), 1 household will avoid anyone developing influenza.
Evidence
Best evidence is from mostly unpublished, industry sponsored RCTs1 (1990s) and 2 systematic reviews.2,3 Results are based on laboratory-confirmed, symptomatic influenza.
Institutionalized seniors.
-Two RCTs examined zanamivir PEP during an influenza outbreak (10 cases or 10% with influenza) using 14 days of zanamivir (10 mg/d) versus rimantadine (N = 385, 98% vaccinated) or placebo (N = 489, 9% vaccinated).
—At 15 days 2.9% of those taking zanamivir had influenza versus 7.4% of those taking rimantadine (statistically significant); 6.3% of those taking zanamivir had influenza versus 9.2% for placebo (not significant).
—Pooled (by authors): 4.6% versus 8.3%; NNT = 27.
-A study of 6 weeks of oseltamivir (75 mg/d) or placebo in 548 (69% vaccinated) patients when influenza was “noted in the community” found influenza at 8 weeks in 0.3% of patients versus 4.4% for placebo; NNT = 25.
For households, 3 clustered (by household) RCTs examined PEP when a household member was diagnosed with influenza-like illness (mean age of contacts 24 to 33 years; < 15% vaccinated; children excluded).
There was no difference in hospitalizations.2,3 Multiple analyses were performed for adverse effects.3 For oseltamivir: psychiatric events (NNH = 95), headache (NNH= 32), and nausea (NNH = 25).2,3 For zanamivir: no difference in treatment trials.3 Limitations include inconsistent outcome definitions and selective reporting.2
Context
Canada stockpiles about 60 million doses (primarily oseltamivir); about 50% expire before use.6
In closed facility outbreaks, guidelines recommend treating the index case, vaccinating the unvaccinated,7 and PEP for the longer of 14 days or 7 days after the onset of symptoms in the last infected person.8 For households, PEP is recommended only if vaccination is contraindicated.7
Implementation
Antiviral PEP is not a substitute for influenza vaccination.8 Vaccine efficacy is determined by seasonal prevalence and vaccine matching. For every 13 long-term care residents vaccinated, 1 fewer will develop influenza.9 Evidence suggests vaccination of health care workers decreases long-term care residents’ mortality.10,11 While 72% of FPs reported being vaccinated, overall health care worker vaccination rates are about 50%.12 Institution outbreaks should be considered when 2 or more residents have influenza-like symptoms within 72 hours of one another.8
Acknowledgments
We thank Dr T. Jefferson for advising us of the location of the unpublished reports.
Notes
Tools for Practice
Tools for Practice articles in Canadian Family Physician (CFP) are adapted from articles published on the Alberta College of Family Physicians (ACFP) website, summarizing medical evidence with a focus on topical issues and practice-modifying information. The ACFP summaries and the series in CFP are coordinated by Dr G. Michael Allan, and the summaries are co-authored by at least 1 practising family physician and are peer reviewed. Feedback is welcome and can be sent to toolsforpractice{at}cfpc.ca. Archived articles are available on the ACFP website: www.acfp.ca.
Footnotes
Competing interests
None declared
The opinions expressed in Tools for Practice articles are those of the authors and do not necessarily mirror the perspective and policy of the Alberta College of Family Physicians.
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