# 4720
Another of the presentations scheduled for the ICEID 2010 conference this week that caught my eye is this one on asymptomatic H1N1 infections among Health Care Workers in Japan.
Asymptomatic Infection of Influenza A(H1N1) 2009 Pandemic Virus among Japanese Healthcare Workers
(note: It’s about 1/3rd down the pdf)
A. Suzuki, T. Odagiri, T. Okada, K. Shimabukuro, A. hno, I. Khandaker, R. Sawayama, Y. Furuse, K. Kawamura , H. Oshitani
First, the abstract, then a few thoughts as to why this one might be particularly important.
Background: One of the limitations of the current influenza surveillance is the detection of
asymptomatic cases, which play important role in disease transmission. It is difficult to detect
asymptomatic cases even with sensitive molecular methods, and a retrospective serological test is the
method to confirm.
The asymptomatic infection among healthcare worker may the big issues in terms of infection control, but its occurrence is unknown. We conducted sero-epidemiology study to elucidate the prevalence of asymptomatic infection of influenza A(H1N1) pandemic virus in a cohort of healthcare workers in Japan.
Methods: One hundred forty healthcare workers, including 25 pediatricians, in 18 private pediatric outpatient clinics in Sendai, Japan, have been participating in the cohort study to monitor
the incidence of influenza infection during pandemic.Serum was collected between 38th and 42nd epidemiological week, the early stage of community transmission in Sendai.
Hemagglutination inhibition (HI) test was used to measure the antibody against pandemic influenza. Purified hemagglutinin of A/California/07/2009 pdm was used as the antigen for HI test, and was kindly provided by National Institute for Infectious Disease in Tokyo, Japan.
All participants were asked to fill the questionnaire on history of influenza-like illness from May 2009 to the day of sampling. Paired-serums of symptomatic laboratory-confirmed cases in this cohort study were used as the reference for HI titer.
Results: Among 123 participants, 33 (27%) had HI antibody titer below 1:10, 39 (32%) had 1:10, 16 (13%) had 1:20, 18 (15%) had 1:40, 11 (9%) had 1:80, 4 (3%) had 1:160, and 2 (1%) had 1:320.
All laboratory-confirmed cases had HI titer higher than 1:160 in convalescence phase serum. Only one case reported to have influenza-like illness after May 2009 and most of them were asymptomatic.By stratification by the job title, medical doctors had highest attack rate.
Conclusion: We were able to confirm the high prevalence of infected cases, as well as asymptomatic cases, among healthcare workers in Japan despite of the fact that most of them wearing surgical or N95 masks.
Alternative approach should be taken place in healthcare facilities to protect workers from influenza infection.
Ok, for those not up to speed on antibody titer values, a seropositive rate is generally defined as ≥ 1:40.
So while this abstract is a bit terse, among the 123 participants of this study, 88 (71.5%) appear to have been seronegative, while as many as 35 (28.5%) had antibody titers that met that criteria for having been infected by pandemic H1N1.
This study is interesting for a number of reasons, and not just for the incidence of asymptomatic cases.
First, while attack rates of 28% have been documented in school age children, most seroprevalence studies to date have come up with significantly lower rate for adults.
In A lower than expected adult Victorian community attack rate for pandemic (H1N1) 2009 the attack rate among adults in Victoria, Australia was estimated at 10%.
And from Clinical Aspects of Pandemic 2009 Influenza A (H1N1) Virus Infection we get this estimation of the attack rate:
After the peak of a second wave of infection in Pittsburgh, the seroprevalence of hemagglutination-inhibition antibody suggested that about 21% of all persons and 45% of those between the ages of 10 and 19 years had become infected
So, a 28% attack rate among adults would seem more than a bit surprising, particularly in Japan, a country renowned for its high standards of hygiene and general phobia of germs.
Granted these are health care workers (HCWs), who are more likely to be exposed to the virus, but that brings us to the second point.
Namely, that in Japan – where infection control is taken very seriously, and compliance with hand washing and mask wearing among HCWs was undoubtedly very high – those measures apparently proved inadequate barriers to infection.
Admittedly, we don’t know how many of these HCWs may have been exposed and infected outside of the workplace, although this study was reportedly conducted early in the transmission stage in Sendai.
With a relatively mild influenza, such as novel H1N1 turned out to be, the results were not disastrous to the health care system.
But had this been a more virulent influenza strain with a high clinical attack rate, infection of 30% of healthcare workers despite the widespread use of N95s and facemasks would have been of serious concern.
Another reason to consider vaccination high among the preventative steps for health care workers.
