HCWs are not acquiring H5N1 strain in Vietnam
HCWs are not acquiring H5N1 strain in Vietnam
Continued viral mutation keeps threat constant
Thanh Liem NT, World Health Organization International Avian Influenza Investigation Team, Vietnam, Lim W. Lack of H5N1 avian influenza transmission to hospital employees, Hanoi, 2004. Emerg Infect Dis 2005. Web: www.cdc.gov/ncidod/EID/vol11no02/04-1075.htm.
No evidence was found of nosocomial transmission of H5N1 avian influenza among 83 health care workers with exposure to case patients in Vietnam, researchers reported.1
The workers were exposed to four confirmed and one probable H5N1 case patients or their clinical samples.
The data suggested the H5N1 viruses responsible for human cases in Vietnam are not readily transmitted from person to person. However, influenza viruses are genetically variable, and transmissibility is difficult to predict, the authors warned.
"While the absolute risk for human-to-human transmission of avian H5N1 viruses may be low at this time, the high case-fatality proportion seen among recent human H5N1 patients demonstrates that the individual consequences of infection are very serious and intensive measures to protect health care workers and laboratory staff against infection remain warranted," the researchers pointed out.
"The risk of human-to-human transmission of H5N1 viruses could increase in the future. Consequently, every H5N1 case should be managed by clinicians and public health professionals with the assumption that human-to-human transmission can occur and that the risk for such transmission is unpredictable," they added.
A number of possible factors may explain the findings, including a lack of infectivity of the patients at the time of admission; the effective use of personal protective equipment and infection control; low sensitivity of the antibody detection method; and lack of susceptibility of the workers or a lack of transmissibility of this particular H5N1 strain.
Most hospital employees reported that they always wore masks — usually N95 respirators — while caring for H5N1 patients.
However, some cases were not immediately diagnosed with avian flu, so some health care workers were exposed in the absence of ideal infection control measures. The case patients primarily were children.
No data are available on the duration of H5N1 virus shedding in children. However, for human influenza virus, viral shedding at high titers is generally more prolonged in children, and virus can be recovered up to six days before and 21 days after the onset of symptoms.
The H5N1 patients in this study were admitted with severe illness three to seven days after onset of symptoms. Two of the patients were treated orally with the nucleoside analogue ribavirin during their admission.
However, the two other confirmed case patients and the probable case patient did not receive antiviral treatment.
"If human infection with H5N1 is associated with viral shedding, these patients would be expected to be contagious during their admission," the authors surmised.
Oseltamivir prophylaxis was not used by any of the staff in this study and therefore did not play a role in protecting health care workers. "Whether the health care workers in the study were protected by cross-reactive immunity to other influenza A subtypes is hard to assess," they concluded.
"One possible explanation for the observation that most confirmed H5N1 case patients are reported in children or young adults is that older adults are protected by cross-reactive immunity from previous exposure to other influenza A viruses."