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Transmission of 2009 Pandemic Influenza to Healthcare Workers

By Amesh A. Adalja, MD, FACP, December 16, 2011

During the early days of the 2009 H1N1 influenza pandemic, before knowledge of the virus was widespread, healthcare facilities that had not instituted influenza control measures were accepting patients infected with the virus, and healthcare workers (HCWs) were exposed. At that time, the CDC, in collaboration with the California Department of Public Health, the San Diego County Health and Human Services Agency, and the Imperial County Public Health Department, catalogued the infection rates among 139 HCWs who were exposed to 6 of the first 8 laboratory-confirmed 2009 pandemic patients in the U.S. The study results have important implications for efforts to minimize the risk of both virus acquisition and virus transmission among HCWs during an influenza pandemic.

ED and Outpatient Exposures

The 6 index patients presented to 1 of 3 healthcare settings between March 28 and April 22, 2009. Those settings included the emergency department (ED) of a tertiary care facility, the ED of a community hospital, or an outpatient clinic (all facilities were located in San Diego County or Imperial County). In those settings, HCWs who were within 6 feet of an index patient, for any period of time, were considered exposed. Included in the study were real-time data from 63 of the 139 HCWs who met this definition.

Data elements covered information about demographics, employment, and work setting as well as information about use of personal protective equipment (PPE). HCWs were queried about the presence of acute respiratory illness—ie, did they have at least 2 of the following symptoms: fever, cough, sore throat, and rhinorrhea within 10 days of encountering an index patient. When HCWs with acute respiratory illness met the more stringent criteria of temperature higher than 37.8C accompanied by cough or sore throat, they were assigned to an “influenza-like illness” subset.

16% Acquisition Rate Among HCWs

Survey data are summarized below:

  • 10 (16%) of the exposed HCWs met study criteria for acute respiratory illness

  • 6 (10%) of those HCWs met criteria for influenza-like illness

  • Of the exposed HCWs with acute respiratory illness, 8 (80%) were absent from work for a median 2 days; 5 (50%) sought medical evaluation, and 2 (20%) received antiviral therapy

  • None of the HCWs who developed acute respiratory infections received post-exposure prophylaxis with an antiviral medication

  • Post-exposure analysis of serum samples indicated that 9 (14%) were seropositive

  • Of seropositive HCWs, 6 (67%) were asymptomatic, and 3 (33%) developed influenza-like illness.

Setting and Role Influence Exposure

Of 9 serologically positive cases among HCWs, 6 (67%) worked in outpatient settings, and 8 (89%) held allied health positions—findings that were statistically significant in a comparison with HCWs who were not infected. Both the setting in which exposure occurred and an HCW’s role influenced acquisition of virus:

  • There was a 2-fold higher attack rate among both allied HCWs and those working in outpatient settings, though neither finding was statistically significant

  • HCWs in outpatient settings were statistically more likely to be seropositive than those working in inpatient settings (32% vs. 7%), a finding that was statistically significant.

Prevention Worked

HCWs in all settings were vaccinated against seasonal influenza at about the same rate, although allied HCWs were less likely to have been vaccinated. Use of personal protective equipment (PPE) was found to be significantly protective¾none of the HCWs who reported use of masks or N-95 respirators was seropositive, while 9 (21%) of those who reported no use of PPE were seropositive. Use of N-95 masks was also associated with lack of respiratory illness symptoms. PPE was used more often in inpatient settings, where fit-testing was performed more often than it was in outpatient settings.

Proper Use of PPE

With the finding of higher infection rates among HCWs in outpatient settings, where PPE is used less often, this study suggests that PPE, and especially N-95 masks, is essential to efforts to protect HCWs. This is true as well for allied HCWs, who may be less likely to don masks. Most important for pandemic management is the finding that infection led to absenteeism, which could produce a significant reduction in workforce at a time when HCWs will be needed most.

In all, these results reinforce the importance of droplet precautions, PPE, and other prevention efforts during an influenza pandemic. The results also suggest the need to stress use of these measures by all HCWs in all settings where daily prevention of nosocomial infection is important.

Reference: Jaeger JL, Patel M, Dharan N, et al. Transmission of 2009 pandemic influenza A (H1N1) virus among healthcare personnel—southern California, 2009. Infect Control Hosp Epidemiol 2011;32:1149-1157.