Published Date: 2004-01-28 23:50:00
Subject: PRO/AH> Avian influenza, poultry vaccines
Archive Number: 20040128.0330
AVIAN INFLUENZA, POULTRY VACCINES
A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Wed, 28 Jan 2004
From: Debora MacKenzie <firstname.lastname@example.org>
[In our posting of 26 Jan 2004, "Avian influenza - Eastern Asia (08)", we
included the information about the local government of Bali, Indonesia,
ordering 50 000 doses of vaccine from China to be distributed to farmers.
We asked whether the said information referred to an H5N1 vaccine. In
response, we have received the following information from Debora MacKenzie,
for which we are very grateful. - Mod.AS]
According to sources at the World Health Organisation, for the past 2 years
or more poultry farmers in southern mainland China have vaccinated their
birds with an H5N1 killed virus vaccine made at the National Veterinary
Research Institute at Harbin, China. In [contrast, in] Hong Kong they use the
[oil-adjuvant] H5N2 Mexico strain vaccine commonly used elsewhere.
Vaccination in the region is said to be very widespread, probably in reaction
to the wholesale destruction of Hong Kong's chickens after the H5N1 cases in
people in 1997.
The H5N1 from the outbreaks in Viet Nam and South Korea has a different
genetic sequence and antigenicity from H5N1 viruses sampled as recently as
2003 in Hong Kong. So it seems probable that the Chinese H5N1 vaccine is
also not a very close antigenic match for the Viet Nam virus. A flu vaccine
will allow flu virus for which it is not a close antigenic match to
continue to circulate at low levels in vaccinated flocks. So the Chinese
vaccine could allow the Viet Nam virus to spread if it is present.
This could continue unnoticed, without mass disease outbreaks to give the
virus's presence away, until it reached an unvaccinated flock. These would
tend to be in smallholdings, and would probably be ducks, because they
traditionally do not get sick with flu and are probably not commonly
vaccinated. So the discovery of H5N1 in a duck smallholding in Guangxi is
The currently circulating H5N1, like the related one that caused an outbreak
in Penfold Park, Hong Kong in 2003, is unique in that it kills ducks as well
as a variety of other birds. This might make it less likely that wild birds
are mainly responsible for carrying the virus over long distances.
It is interesting to speculate what selective pressures an H5N1 virus
circulating at subclinical levels in very large numbers of partially-
immunised chickens might be subject to, and how that might relate
to the emergence of the current outbreaks.
[The speculation raised by Debora MacKenzie is compelling. An A/H5N1 strain
was isolated in S. Korea in May 2001 from imported Cherry Valley Pekin duck
meat. The meat had been processed at a food factory in Shanghai, mainland
China, brought from farms located in the Shanghai region.
The isolate, designated DK/Anyang/AVL-1/01, was studied by Tumpey et al
(Journal of Virology, June 2002, p. 6344-6355, Vol. 76, No. 12). They
reported that, following intravenous or intranasal inoculation, the said
virus was highly pathogenic and replicated to high titers in chickens,
while no clinical signs of disease were observed in inoculated ducks.
However, the infectious virus could be detected in lung tissue, cloacal, or
oropharyngeal swabs of the inoculated ducks. Virus inoculations in mice
caused weight loss and resulted in 22 to 33 percent mortality. The said
results led the writers to the conclusion that highly pathogenic H5N1
influenza virus strains "are still circulating in China and may present a
risk for transmission of the virus to humans."
Later (see 20040127.0322), A/H5N1 strains were isolated from duck's meat
exported to Japan (May 2003) and from tracheal swabs taken before the
destruction of 6 smuggled ducks in Taiwan (December 2003). Data on the
pathogenicity of those strains could provide additional insights regarding
the epidemiology of the current pandemic.
In 1998, an American company announced the production of an experimental
batch of an H5N1 human vaccine, and -- according to their communication
(see 19980205.0225) -- "delivered more than 1000 doses of the new vaccine
to the NIH for use in trials". Information on the fate of the said
vaccine, and its ability to protect against the current strain(s), are of
Hong-Kong's strategy to control avian influenza a in 2003, including the
vaccination issue, was explained in their press release:
<http://www.info.gov.hk/gia/general/200301/17/0117191.htm>. - Mod.AS]