Oseltamivir prophylaxis worked, but drug may pose reinfection risks

Treatment with an antiviral agent may decrease a patient's protection against reinfection in a subsequent pandemic, although a combination of early detection and “ring” protection can truncate epidemics in close quarters.

Epidemiology and containment strategies from the 2009 influenza A (H1N1) pandemic, especially regarding the use of oseltamivir (Tamiflu), were assessed in two new studies from Hong Kong and Singapore, published in the June 10 New England Journal of Medicine.

The Hong Kong study included 99 patients who were seen at outpatient clinics for acute respiratory illness and who had positive results on a rapid test for influenza A. In addition, nasal and throat swabs were collected from all members of the patients' households during three visits over the next seven days. Samples were also taken from a subgroup of patients to test for antibody responses to both the pandemic and seasonal influenza A viruses.

Overall, the epidemiology of the viruses appeared similar; secondary attack rates, viral shedding, and the course of illness showed insignificant differences between the pandemic disease and the seasonal one. However, the study did find one potential difference between these strains. Almost half (44%) of the patients received oseltamivir, and those patients had reduced antibody titers to the pandemic strain compared to patients who hadn't received drug treatment.

Combined with some recent case reports, this finding could indicate that treatment with an antiviral agent may decrease a patient's protection against reinfection in a subsequent pandemic, the study authors said. This could be true of pandemic influenza but not seasonal influenza because immune systems have already been primed to respond to seasonal flu by exposure to previous, closely related strains, the authors explained.

The Singapore study analyzed the effectiveness of oseltamivir as a prophylactic strategy during H1N1 outbreaks in military camps. During four outbreaks, a total of 1,110 personnel received prophylaxis. Three out of the four outbreaks showed significant reductions in infection rates after the prophylaxis was instituted. The anti-flu effort also isolated infected patients and segregated affected military units.

The study authors concluded that early detection (which was accomplished in three of the outbreaks through education and daily monitoring for symptoms) and “ring” prophylaxis (defined as “geographically targeted containment”) with antiviral drugs can effectively truncate the spread of an epidemic.

The strategy may be appropriate in areas where vaccine supply is limited or unavailable, or in situations where the vaccine is a poor match with the circulating virus. Aggressive prophylaxis could be particularly useful in long-term care facilities, schools, prisons or military camps, the study authors said. However, an accompanying editorial cautioned that compliance and oseltamivir resistance could pose problems. Vaccination should remain as the primary prevention tool for influenza, the editorialist wrote.