Current Trends Antigenic Variation of Recent Influenza
A(H1N1) Viruses
Antigenic analysis of recent influenza A(H1N1) viruses,
including
those reported from several countries in Southeast Asia (1), has
detected antigenic drift from the previously prevalent strain
related
to A/Chile/1/83(H1N1). The antigenic variation has been seen both
in
reciprocal hemagglutination-inhibition tests using animal antisera
and
in comparisons of antibody levels against the newer variants in
persons immunized with vaccine containing A/Chile/1/83 antigen.
Animal antiserum to A/Chile/1/83 is less effective in
inhibiting
the newer A(H1N1) variants than in inhibiting A/Chile/1/83-like
viruses, although the degree of reactivity varies among isolates of
the newer A(H1N1) variants (Table 3). In contrast, antisera
prepared
against representative new isolates (e.g., A/Singapore/6/86,
A/Taiwan/1/86) react well with all other recent isolates but
extremely
poorly with A/Chile/1/83. In tests with animal antisera, the new
variants also differed from other variants that have cocirculated
with
A/Chile/1/83 since 1983-1984 (e.g., A/Victoria/7/83,
A/Dunedin/27/83).
Studies of antibody response in recipients of last year's
influenza vaccine containing A/Chile/1/83 antigen demonstrate the
difference of the new variants (Table 4). Approximately 80% of
adult
vaccinees developed titers to A/Chile/1/83 of 160 or higher and had
a
postvaccination geometric mean titer (GMT) of 320. In contrast,
only
15%-30% of the vaccinees developed the same titers to the new
variants, and the postvaccination GMT was about sixfold lower.
However, since postvaccination titers of 40 or greater have been
associated with reduced influenza infection and illness, it is
possible that the A/Chile/1/83 antigen in the 1986-1987 trivalent
vaccine will provide at least partial protection against the new
variants.
Through July 1986, influenza A(H1N1) variants had been detected
among virus isolates from Hong Kong, Malaysia, and New Zealand;
from
an imported case in England believed to have originated in India;
and
among viruses from a few outbreaks and sporadic cases of influenza
A(H1N1) that occurred in Japan last winter. The presence of an
A(H1N1) variant has also been reported from the Soviet Union, where
last winter's epidemic was primarily caused by influenza B and
influenza A(H3N2) viruses. Recently, influenza A viruses, some
confirmed as A(H1N1) strains, have also been reported in Australia,
Thailand, American Samoa, and Palau, Micronesia. Influenza
outbreaks
in Palau and elsewhere in the U.S. Pacific Trust Territories are
presently being investigated.
Reported by National Influenza Centers in collaboration with the
World
Health Organization, Geneva; WHO Collaborating Centre for
Influenza,
Influenza Br, Center for Infectious Diseases, CDC.
Editorial Note
Editorial Note: After the World Health Organization (WHO)
Collaborating Centres for Influenza detected variants of influenza
A(H1N1) viruses among isolates submitted for reference analysis
from
influenza outbreaks in the People's Republic of China, Malaysia,
Singapore, and Taiwan, data were reported indicating that related
strains had been detected at the end of an influenza A(H3N2)
epidemic
in Japan last winter. The most recent reports suggest that the
variant may have been introduced into other countries widely
separated
in the Pacific Basin from Southeast Asia (e.g., Australia, New
Zealand, and several islands in the U.S. Pacific Trust Territories)
and possibly India. Reports of outbreaks in Southeast Asia
indicate
primarily children and young adults have been affected, suggesting
that the general epidemiologic characteristics of the new variant
are
similar to those of other type A(H1N1) variants occurring since
1977.
Based on the available reports, WHO has suggested that national
health authorities consider the addition of the new type A(H1N1)
virus
to other strains already recommended for incorporation in 1986-1987
influenza vaccines, either as an extra component in current
trivalent
vaccines or as a monovalent vaccine (2). In the United States, the
vaccine production schedule permits manufacture only of a
supplemental
monovalent vaccine with a new type A(H1N1) virus. After
consideration
of available data, the U.S. Public Health Service is recommending
production and use of such a vaccine to maximize protection against
the new variant, particularly for high-risk children and young
adults
who appear to be most susceptible to illness caused by current type
A(H1N1) viruses (3). If outbreaks of influenza-like illnesses
occur
in the United States early this fall, prompt laboratory diagnosis
will
be necessary to establish whether they are due to influenza A
viruses
and to guide appropriate use of the available antiviral agent,
amantadine hydrochloride (4).
ACIP. Prevention and control of influenza. MMWR
1986;35:317-26,
331.
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