Influenza epidemics are responsible for massive disruption to
industry, and for a significant number of deaths, particularly in
the elderly and the very young. At present, treatment of
influenza is entirely symptomatic. Salicylates should be avoided
in children because of the link with Reye's syndrome. 2
compounds, amantidine and ribavirin, with antiviral activity
against influenza have been identified and may be of value.
Amantidine - this compound inhibit the growth of
influenza viruses in cell culture and in experimental
animals. Amantidine is only effective against influenza
A, and some naturally occurring strains of influenza A
are resistant to it. The mechanism of action of
amantadine is not known. It is thought to act at the
level of virus uncoating. The compound has been shown to
have both therapeutic and prophylactic effects.
Amantidine significantly reduced the duration of fever
(51 hours as opposed to 74 hours) and illness. The
compound also conferred 70% protection against influenza
A when given prophylactically. Amantidine can
occasionally induce mild neurological symptoms such as
insomnia, loss of concentration and mental
disorientation. However, these symptoms quickly developed
in susceptible individuals and cease when treatment is
stopped. The therapeutic and prophylactic activity of
amantidine is now generally accepted and numerous
analogues of this compound have been prepared.
Rimantadine is not as effective as amantadine but is less
toxic. Prophylaxis with 200mg of amantadine per day for 5
to 6 weeks or for the duration of the influenza A
outbreak is not recommended for all persons. However,
elderly persons with chronic underlying disease,
institutionalized persons, staff and patients in
hospital, close contacts of an index case, and patients
who cannot receive influenza A vaccine due to sensitivity
to egg protein may benefit from prophylaxis. Amantadine
can also be used for therapy of uncomplicated influenza A
infections. The recommended dose is 200mg for 5 days.
Rimantadine may be used in place of amantadine for
prophylaxis and the treatment of uncomplicated influenza
A infections.
Rimantidine - this compound is similar
to amantidine but has fewer side effects. It is approved
by the FDA for the treatment and prophylaxis of influenza
A infection in persons one year or older. It should be
used for uncomplicated influenza A infections only since
it is thought to be less effective than amantidine.
Amantadine and rimantadine resistant viruses are readily
generated in the laboratory. Resistance has been linked
to changes in the M2 protein. To date, the emergence of
resistant influenza A has been documented primarily in
young children undergoing therapy with rimantadine. The
resistant viruses had been transmitted and caused
influenza. The universal susceptibility of all types of
naturally occuring influenza A isolated from man and
animals suggests that resistance will be found only in
individuals treated with the drug. The reason for the
natural selection of the susceptible phenotype of
influenza A in nature is not known.
Zanamivir - the rational approach to
drug design has led to the design of several potent
inhibitors of influenza neuraminidase. Zanamivir was the
first neuraminidase inhibitor available for clinical use
and is effective against both influenza A and B. Because
of its poor bioavailability, zanamivir must be
administered by inhalation. Zanamivir had been shown to
be effective and devoid of significant side effects in
clinical trials. It is now approved by the FDA for use as
treatment for influenza A and B in persons 12 years or
older but not for prophylaxis.
Oseltamivir - oseltamivir is another
neuraminidase inhibitor but unlike zanamivir, it can be
given orally. Like zanamivir, it had been shown to be
effective and devoid of significant side effects in
clinical trials. It is approved by the FDA for use as
treatment for influenza A and B in persons 18 years or
older. It is also approved for prophylaxis in persons 13
years or older. Its lack of side effects would make
particularly attractive in a family setting although its
higher cost compared to amantidine and rimantidine should
be taken into account.
G. Prevention
Vaccines against influenza have been around for 50 years.
Despite this, the efficacy of influenza vaccines is still
questioned, and the ability of vaccines to limit epidemic
infection has not been proven.
1. Immunity to Influenza - the results of challenge
studies indicated that immunity is induced by the host responses
to the virus haemagglutinin (HA) and to neuraminidase (NA).
Antibody against HA is the most important component in the
protection against influenza viruses. In addition to conferring
relative protection against infection, serum HI is reported both
to reduce the severity of infection and decrease virus spreading
in infected persons. Serum anti-neuraminidase Ab has also been
shown to contribute protection against influenza infection. A
generally held view is that serum HI antibody is more important
in determining immunity than anti-neuraminidase antibody. It is
clear that an influenza vaccine must contain both HA and NA
antigens in a form which will stimulate the production of
neutralizing antibody, local IgA antibody and possibly cellular
immunity.
2. Types of vaccine
Whole virus vaccines - whole inactivated virus
vaccines were the first influenza vaccines to be
produced. The currently circulating strain of influenza
is inoculated into embryonated eggs, harvested 2-3 days
later and inactivated. this vaccine confers protection in
60-90% of vaccinees and the protection lasts for 1-5
years, depending on the vaccine strain and the age of the
vaccinee. However, the subsequent infecting virus may
show slow antigenic drift and the vaccine induced
antibody will be less effective in conferring protection
against the new strains.
Split virus vaccines - Because of the high
incidence of reactions seen in vaccinees given whole,
inactivated virus vaccine, attempts have been made to
produce a vaccine which is less reactogenic but
conserving immunogenicity. Split vaccines were prepared
inactivated particles disrupted with detergents. These
vaccines have been shown to induce fewer side effects in
the vaccinees and are just are immunogenic as whole virus
vaccine. Whole virus vaccine should not be used in
children.
Subunit virus vaccines - subunit vaccines have
been prepared which contained only the HA and NA
antigens. These are used in aqueous suspension or may be
absorbed to carriers such as alhydrogel. Volunteers given
subunit vaccines experienced fewer reactions than those
given whole virus vaccines and those given aqueous
vaccine experienced fewer reactions than those given the
absorbed subunit vaccine. therefore, the best vaccines
available at present are the aqueous subunit vaccines,
although some authorities have questioned the
effectiveness of subunit vaccines.
Live attenuated vaccines - there is experimental
evidence that immunization with live, attenuated
influenza virus vaccines induce a solid immunity than do
inactivated vaccines. Normal methods for attenuation,
such as repeated passages and temperature adaptation
require a long period to complete, and probably too long
for the vaccine to become available for immunization
against the current influenza strain. To circumvent this
problem , already attenuated strains have been mixed with
wild-type virus to produce recombinants which contain the
RNA fragments which code for wild-type HA and NA, and all
the other genetic material form the attenuated strain.
These recombinants can be produced relatively quickly.
When given intranasally, produced few side effects.
Although research to develop live attenuated vaccines has been
pursued for 20 years, basic problems remains particularly in the
area of purification. The vaccine must also be shown to be
attenuated and safe. It is estimated that if the safeguards are
to be satisfied, 2 years would be needed for the development of
an attenuated vaccine. This makes their development impractical ,
since by the time the vaccine virus can be made available, the
epidemic strain against which the vaccine has been prepared would
have disappeared. For an attenuated vaccine to be a practical
proposition, the development time must be down to 6-9 months.
3. Recommendations - At present, no live attenuated
vaccine is available for general use. The vaccines that are
currently available are produced from virus grown in embryonated
eggs. The aqueous subunit vaccine is the most acceptable
formulation. These vaccines produce few reactions and confer
protection in 60-90% of vaccinees. Vaccination is recommended for
the elderly and individuals at risk for severe infection. In
addition, key personnel n industry and social and medical
services. It is highly debatable whether the vaccine should be
given to the general population. Influenza immunization is
strongly recommended for adults and children with any of the
following:
1. Chronic respiratory disease
2. Chronic heart disease
3. Chronic renal failure
4. Diabetes mellitus and other endocrine disorders
5. Immunosuppresion due to disease or treatment
Immunization is also recommended for residents of nursing
homes and old peoples' homes and other long stay facilities where
rapid spread is likely to follow. Two types of vaccines are
available in the UK; "split virus vaccines", and
"surface antigen" vaccine which contains highly
purified HA and NA antigens prepared from disrupted virus
particles. Both vaccines are suitable for use in children.
4. WHO Influenza Surveillance Program
The WHO has a network of around 110 influenza centres
worldwide that regularly submit new influenza isolates to the 4
WHO collaborating centres (US, UK, Japan, Australia) for
analysis. The aim is to detect new and potentially
dangerous strains of influenza at the earliest moment so that
measures can be enacted in the event of a pandemic. The strains
used in current influenza vaccines are supplied to the vaccine by
the WHO. It is normally a trivalent vaccine: one H3N2, one H1N1
and one influenza A sutype. The sutypes selected are those that
are normally the most antigencally diverse strains considered to
have to greatest epidemic potential.
H. The H5N1 avian influenza outbreak in Hong Kong 1997
In the latter half of 1997, an outbreak occurred in Hong Kong
whereby 18 persons were infected by an avian influenza A,
serotype H5N1. Of these 6 died, and 3 others were severely ill.
The source of the outbreak was infected chickens and the outbreak
stopped after all the chickens were slaughtered in the territory.
Large-scale serological studies carried out showed that workers
in the poultry industry were particularly at risk of infection
although none complained of any symptoms. There was evidence of
limited human to human transmission. It was postulated that the
strain of avian influenza involved was unusually virulent; it had
multiple basic amino acids near the cleavage site of the
haemagglutinin protein, which as a result may render the
haemagglutinin susceptible to a wider range of proteases. Since
that outbreak, no more cases have occurred. In 1999, there were
reports of human infections by avian influenza A H9N2 in Hong
Kong and in Mainland China. However, all these cases were very
mild and it is thought that the virus was unlikely to pose a
large public health risk.
