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Patients with influenza A H1N1 may be contagious longer than initially thought
Infection control experts based initial seven-day precautionary isolation measures for patients with novel influenza A H1N1 on knowledge of seasonal influenza viral shedding patterns, but data presented at the American Society for Microbiology’s 49th Interscience Conference on Antimicrobial Agents and Chemotherapy this week suggests that patients with H1N1 may actually shed live virus longer.
“We cannot say whether the [live viruses] are really contagious, but they have the potential to be,” Gaston De Serres, MD, PhD, of the National Institute of Public Health in Quebec, said at the meeting.
De Serres and colleagues from several other sites in Canada used polymerase chain reaction to diagnose influenza in 44 household contacts of patients with confirmed influenza A H1N1.
They performed PCR and viral cultures again eight and 10 days following disease onset and noted that 75% of patients tested with standard influenza PCR and that 43% of those tested with an H1N1-specific PCR still had virus present on day eight. To determine whether or not the virus was live, the researchers performed an additional viral culture with positive results in 30% of patients. No patients had live virus present on day 10.
Although the researchers determined that 30% of patients were still shedding virus one week after disease onset, De Serres emphasized that they were unable to determine exactly how contagious patients actually were because the amount of live virus needed for H1N1 to be transmissible remains unknown.
He also stated that this proportion is probably an overestimate of what physicians are likely to see in practice because they have yet to determine if household contacts with respiratory syndromes truly had influenza H1N1 or another virus.
He said conservative estimates put the bare minimum of patients with H1N1 who are still infected with virus on day eight at about 8%. But he added that the data may have important implications for infection control policies in the United States and Canada.
“There have been suggestions that after a patient’s fever subsides [he/she] may go back to work or school. In a substantial portion of patients with fever generally lasting one to four days, they might still be contagious. Immediately returning to school or work would likely cause further transmission of the disease,” De Serres said.
He said policy makers should consider this data along with other data presented at ICAAC that indicate that the patients with H1N1 have greater viral loads than those with the seasonal influenza virus.
“We should not relax the isolation period if we want to reduce transmission,” De Serres said. – by Nicole Blazek