Some patients with 2009 pandemic influenza A(H1N1) shed live virus a few days longer than commonly occurs with seasonal flu, according to a Canadian study with 100 patients.
The public health implications of the finding aren't clear, Dr. Gaston De Serres said during a press briefing at the annual meeting of the Interscience Conference on Antimicrobial Agents and Chemotherapy.
The results show that it's not enough to isolate people infected with pandemic H1N1 flu for just a couple of days after they become sick or until their fever resolves. People “may be tempted to reduce their time at home [when infected by H1N1], but our results show that would not be wise,” said Dr. De Serres, a medical epidemiologist at the National Public Health Institute of Quebec.
The study focused on 43 patients with symptomatic flu who were culture positive for the pandemic virus. In this group, eight (19%) remained culture positive 8 days after their symptom onset. In contrast, all patients with seasonal flu are routinely culture negative a week after symptom onset. “We can say that H1N1 appears to be shed longer [than seasonal flu] but not much longer,” said Dr. De Serres, who also is professor of epidemiology at Laval University, Quebec. All 43 H1N1 patients in the study were culture negative 10 days after symptom onset.
Another 57 family members of these cases had concurrent flulike symptoms, but all 57 were culture negative the first time they were tested. Adding these 57 to the first 43 produced a total of 100 patients apparently infected with H1N1, of whom 8 were culture positive a week after their illness began, establishing a minimum 8% rate for the persistence of H1N1 shedding beyond 1 week.
Dr. De Serres cautioned that the findings don't mean that all eight patients remained contagious at day 8. Contagion requires more than just shedding live virus; it also requires transmission of an adequate virus dose.