Seroprevalence

Toner · Osterholm · Nichol · Perdue · Dowell · ECDC · WHO table · Country Studies

The issue of seroprevalence, the number of people who have antibodies to H5N1, is important because it can address these critical questions: Are there many mild cases that have not been detected? Has human to human spread of H5N1 influenza occured more frequently than current reports would indicate?

According to Eric Toner of the Clinicians’ Biosecurity Network,

“The true incidence of H5N1 infection and its full clinical spectrum is critical information that must be made available. Hopefully answers to many of these question will become clear in the near future through serological studies, continued collection and sequencing of viral specimens and detailed analysis of clinical cases.”

In a recent email exchange with ER DOC, a commentator on the Flu Wiki discussion board, Dr. Osterholm suggested that with regard to H5N1 seroprevalence studies:

“A lot has been done –just NOT published. I’m aware of over 4,000 individuals tested in conjunction with H5N1 case environments in Indonesia, Viet Nam, Thailand and Hong Kong. An extremely low rate of positivity unless a recognized clinical case. WE NEED THESE DATA PUBLISHED ASAP.”

From Science, 20 January 2006

A broad serosurvey-among patients’ conctacts, those exposed on the job, and the community in general would be a better guage of the true extent of H5N1′s spread. Several such studies have been carried out in Asian countries, but the results have not been published, to the frustration of many flu experts. “Seroplogical studies are often very hard to get out of countries-they’re politically sensitive, even in Western countries,” says Nichol. But from what he has heard about completed H5n1 serosurveys, they do not point to widespread infection beyond the recorded H5N1 cases, he says.

Angus Nichol of the European Centre for Disease Prevention and Control is quoted.

From UPI, Jan 31 2006

Since the initial signs of human infection, scientists in Indonesia, Vietnam and Thailand have been studying those around the people who became infected, looking not only for signs of infection and increased transmissibility, but also for antibodies that could help to explain why some people became infected while others remained healthy.

“The evidence for widespread asymptomatic infections is just not there,’‘ said Michael Perdue, a World Health Organization scientist working on the global influenza program. “The (more recent) studies that have been done, one of the reasons frankly that I think they haven’t been followed up on, is they haven’t found many positives. You don’t get too excited about all negative serology (blood work).”

Dr. Scott Dowell, director of global disease detection and preparedness at the U.S. Centers for Disease Control in Atlanta, agrees: “If there was mild or asymptomatic H5 infection and that was relatively common and the severe cases we’re picking up were the tips of the iceberg, then if you look at family members or hospital contacts or cullers, you would expect to be seeing some of these.

See also Q&A with ECDC on seroprevalence here (link under repair).

Below are some of the available seroprevalence studies for H5N1.

From NEJM, Avian Influenza A (H5N1) Infection in Humans The Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5 N Engl J Med. 2005 Sep 29;353(13):1374-85:

  • Cambodia
    • Cambodian study suggests mild bird flu cases aren’t going undetected
      Buchy and his colleagues reported on work they conducted last spring in a Cambodian village that had H5N1 outbreaks in poultry and where one of that country’s four recorded cases lived. (All four Cambodians known to have been infected with the virus died.) The researchers tested blood samples from 351 of the villagers looking for the antibodies to the virus that would be proof of mild or asymptomatic infections. They found no signs of additional infections - this despite the fact that many of the villagers had significant exposure to infected poultry.
  • China
    • Cohort studies on avian flu
      Studies of the prevalence of H5N1 antibodies in health care workers exposed to H5N1 infected patients were conducted in Hong Kong in 1998. Of 547 health care workers, 10 were reported positive. One healthcare worker positive for H5N1 “…with no reported poultry exposure had a four-fold rise in antibody titer and a temporally-related febrile illness after exposure to a case-patient. The case suggested that influenza A (H5N1) was transmitted from at least one patient to a healthcare worker.”
    • Clinicians’ Biosecurity Network
      “Nature quoted Yi Guan of Hong Kong as saying that that his team tested 4,000 people in southern China in 2001 and found no H5 antibodies.”
    • Times (UK)
      Referring to a son-father H2H case written up in Lancet: Furthermore, the scientists found that 91 people had close contact with one or both of the infected men, and yet the virus was passed on only once.
  • Vietnam
  • Thailand
    • Seroprevalence of Anti-H5 Antibody among Thai Health Care Workers after Exposure to Avian Influenza (H5N1) in a Tertiary Care Center
      “After the initial atypical presentation of a patient with avian influenza (H5N1) infection, paired acute-phase and convalescent-phase serum samples obtained from 25 health care workers (HCWs) who were exposed to the patient were compared with paired serum samples obtained from 24 HCWs who worked at different units in the same hospital and were not exposed to the patient. There was no serologic evidence of anti-H5 antibody reactivity or subclinical infection in either of the groups”.
    • Clinicians’ Biosecurity Network
      “Dr. Kumnuan Ungchusak, Director of the Bureau of Epidemiology of the Thai Ministry of Health has reported (personal communication) that a study of 800 contacts of H5N1 victims has shown a seropositivity rate of less than 1%.”
  • Pakistan
    • Poultry workers test negative for bird flu
      The H5 strain was detected at two poultry farms in Charsadda and Abbottabad in NWFP. Health Ministry and the World Health Organisation (WHO) teams collected blood samples from 18 poultry workers which were tested at the National Institute of Health (NIH).
  • India
    • Bird flu: All human samples test negative
      200-odd samples from Maharashtra
      According to the official, all 18 blood samples were free of bird-flu. He said the samples could have been sent to London but the NIH’s testing procedures were reliable and there was no need to send the samples abroad. The Health Ministry had also made arrangements to ensure the supply of Tamiflu, an antiviral drug used in the treatment of influenza.
  • South Korea
    • Korean Bird Flu Infections Confirmed
      According to the KCDC, the cases were confirmed after it carried out an antibody test on the blood serum of 318 soldiers and workers who took part in a poultry cull from December 2003 to March 2004. Four tested positive for H5N1, a mutated bird flu strain.
Page last modified on October 12, 2009, at 10:30 AM by pogge