Thursday, April 1, 2010

Entry Screening to Delay Local Transmission of 2009 H1N1

After the WHO issued the global alert for 2009 H1N1, many national health agencies began to screen travelers on entry in airports, ports and border crossings to try to delay local transmission.

The authors of this study reviewed entry screening policies adopted by different nations and ascertained dates of official report of the first laboratory-confirmed imported H1N1 case and the first laboratory-confirmed untraceable or ‘local’ H1N1 case.

Screening policies adopted by countries included:

  • Temperature checks onboard aircraft prior to disembarkation.
  • Health declaration forms collected from every traveler or all travelers from countries identified with confirmed H1N1 cases.
  • Arriving travelers were observed by alert staff for influenza symptoms (e.g. cough).
  • Travelers were scanned for elevated body temperature by thermal scanners.
Of the nations looked at in the study, two (China and Japan) implemented all four tools. Five nations did not implement any of the four. Overall, implementation of the four tools alone or in combination were associated with on average additional 7-12 day delays in local transmission compared to nations that did not implement entry screening.

The authors' results suggest that entry screening did not lead to substantial delays in local H1N1
transmission which is consistent with theoretical results from previous modeling studies and findings from previous pandemics. While longer delays in local transmission to the summer in countries in the Northern hemisphere could have substantially aided pandemic mitigation, due to seasonal factors and school vacations leading to lower peak attack rates, the observed delays in the present pandemic suggest entry screening provided around 1-2 weeks of additional time for
preparation and planning.

While this study focused on the impact of entry screening, some nations also implemented other containment and mitigation measures, such as isolation of suspected or confirmed cases, quarantine of their contacts with or without antiviral chemoprophylaxis, school closures or other social distancing measures, and public health campaigns to improve hygiene. Most nations enhanced their influenza surveillance. If countries that expended greater effort into entry screening also had more effective containment and mitigation measures in the general population, these might have led us to overestimate the effect of entry screening.

Conversely, if countries that expanded greater effort into entry screening also tended to have better influenza surveillance and were able to identify local transmission earlier, we may have underestimated the effect of entry screening. Other differences between countries in laboratory capacity and availability of public health resources may also have confounded our evaluation, and all of these factors are limitations of our study.

Previous mathematical modeling studies have questioned the value of entry screening, since it could only delay rather than prevent local epidemics. However, most models assumed that source countries would conduct exit screening and infectious cases would not travel. In such a scenario it is not surprising that entry screening would add little benefit, since most journeys are shorter than the average 1.5-2 day incubation period for influenza A virus infections. Screening is unlikely to identify 100% of ill travelers, while some might use antipyretics to reduce a fever prior to passing through thermal scanners, or fail to report symptoms on declaration forms. Many individuals with subclinical or asymptomatic illness would not be identified, and could initiate outbreaks after arrival. In Hong Kong, only one third of confirmed imported H1N1 cases were identified through screening on entry to Hong Kong, the majority of imported cases were
identified through the local health care system after arrival. A similar experience has been reported in Singapore.

Nevertheless, entry screening could act as a deterrent to traveling when ill or lead to other indirect benefits such as improving public awareness of the pandemic. For entry screening to be successfully employed, substantial resources are required to identify the small fraction of travelers who may have H1N1 infection. Further resources may be needed to isolate identified cases, and trace and quarantine close contacts.

The authors go on to discuss other caveats that may limit the accuracy of their model.

Source: BMC Infectious Diseases

No comments: