Prompt Prophylaxis Is Key After Influenza A Outbreaks

A new study of influenza A outbreaks at long-term care facilities (LTCFs) has shown that when amantadine chemoprophylaxis was started within 5 days of the onset, outbreak duration, incidence rates, and case-fatalities rates all fell significantly.

Researchers at Columbia University (New York, NY, USA) analyzed data from 52 influenza A outbreaks that took place in LTCFs in New York City (NY, USA) from 2001 to 2004, and involved the use of amantadine chemoprophylaxis. An outbreak was defined as a single laboratory-confirmed case of influenza A or a cluster of at least two cases of influenza-like illness. Overall, 27 of the facilities began chemoprophylaxis within 5 days of outbreak onset and 25 started chemoprophylaxis after 5 days.

The researchers found that after accounting for influenza season year, facility bed capacity, and percentage of residents vaccinated against influenza, initiation of amantadine chemoprophylaxis within 5 days of outbreak onset decreased the outbreak duration from 18.3 to 6.7 days. Prompt chemoprophylaxis was also associated with lower incidence rates (6.2 versus 10.5 cases per 100 residents) and case-fatality rates (0.45 versus 3.3 deaths per 100 residents with influenza A). The study was published in the July 1, 2008, issue of Clinical Infectious Diseases.

"These findings make sense as the earlier an outbreak of influenza is identified and barriers to transmission are introduced, we would expect fewer new cases, and a less severe and extensive outbreak,” said lead author Marcie Rubin, MPH, MPA, of the department of sociomedical sciences. "In a future study, it would be important to see whether our findings were replicated in long-term care facilities where neuraminidase inhibitors were used for chemoprophylaxis during outbreaks of influenza.”

Amantadine is formally known as 1-aminoadamantane, and consists of an adamantine backbone that is substituted at one of the four methyne positions with an amino group. The compound is used both as an antiviral and as an anti-Parkinson drug. Amantadine is not currently recommended for chemoprophylaxis because of widespread resistance in circulating influenza strains. The United States Center for Disease Control (CDC, Atlanta, GA, USA) found an alarmingly high resistance rate of 92% in one major flu strain in 2005, and issued an alert to doctors not to prescribe amantadine any more for that season. Among some Asian countries, A/H3N2 and A/H1N1 resistance to amantadine has reached 100%.


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