VAIL, COLO. – A universal influenza vaccine is not a pipe dream.
"There is a really big push for this now. It’s a major goal," Dr. Wayne Sullender observed at a conference on pediatric infectious diseases sponsored by the Children’s Hospital Colorado.
The impetus for development of a universal influenza vaccine is that influenza still poses a major public health threat despite the widespread availability of current vaccines. Worldwide, roughly 1.4 million children die of pneumonia each year, more than from malaria, AIDS, and measles combined. It has been estimated that each year up to 112,000 children under age 5 die of influenza-associated acute lower respiratory tract infection, with 99% of the deaths occurring in developing countries.
A universal influenza vaccine could render obsolete the current costly, time-consuming, and uncertainty-ridden process of reformulating flu vaccines from year to year based upon expert consensus as to what the epidemic strains are most likely to be in the next flu season. This is a guessing game, and vaccine efficacy is reduced in seasons where the match isn’t good.
Also, a universal vaccine could conceivably protect against highly pathogenic pandemic influenza viruses, such as the swine flu H3N2 or the even more lethal avian H7N9 influenza virus. And even if a universal influenza vaccine wasn’t fully protective against threatening pandemic strains, it could perhaps prime vaccine recipients so they are no longer immunologically naïve, explained Dr. Sullender, an infectious diseases expert who is a visiting professor of pediatrics at the University of Colorado, Denver.
All of the universal flu vaccines in clinical development employ various highly conserved regions of influenza virus target antigens. In focusing on these targets shared by different influenza virus subtypes, the goal is to develop vaccines that protect against seasonal influenza, even as the viruses engage in their relentless antigenic shift and drift, as well as to provide immunity against emerging pandemic strains having the potential for rapid spread and high mortality throughout the world.
Among the novel strategies for development of a universal influenza vaccine being pursued in laboratories around the world, one of the most promising in Dr. Sullender’s view involves stimulation of anti-M2e antibodies. M2 is a proton-selective ion channel that plays a key role in virus assembly. M2 is found on the surface of virus-infected cells. Its advantage as an antigen is that its sequence is virtually the same in every influenza virus isolated since the 1930s. Natural infection doesn’t stimulate much of an antibody response to M2. Yet even though M2e antibodies are not virus-neutralizing, it appears they are able to kill influenza virus by other mechanisms.
Another active area involves antibody responses to highly conserved epitopes on hemagglutinin. A region of vulnerability has been identified in the stem region of hemagglutinin, the viral spike. If the amino acids in this stem antibody binding site prove to be so important to the structure of hemagglutinin that the virus can’t tolerate change there, then the virus wouldn’t be able to adapt to and mutate away from a vaccine targeting this site via stimulation of neutralizing antibodies. Such a vaccine could very well be a universal influenza vaccine.
In addition, a novel epitope has been identified on the globular head of the H1N1 influenza virus hemagglutinin. Investigators have isolated a human monoclonal antibody that recognizes this epitope and neutralizes many different H1N1 strains. This could eventually lead to production of vaccines that incorporate protection against the severe H1N1 flu.
With regard to the avian-origin H7N9 influenza A virus that emerged last winter in China, Dr. Sullender commented, "This one is pretty scary." First estimates are that one-third of people hospitalized with the infection died. However, less severe cases were probably underrecognized, and it’s unlikely the death rate will remain this high.
The human-to-human transmission rate of H7N9 is low. Still, there are several reasons for concern about this virus. Although the pathogenicity in birds is low, the virus appears to have enhanced replication and virulence in humans. And H7N9 is already resistant to amantadine. Moreover, cases of resistance to oseltamivir and zanamivir have been reported.
The potential for mayhem due to H7N9 is such that vaccine development efforts are already underway. Among infectious respiratory disease experts, all eyes are on the coming flu season in Asia and what role H7N9 will play.
"Time will tell whether this will be just another story that comes and goes with influenza, or it becomes a more long-lasting problem," he said.
Experts all agree that it’s not a matter of "if’" another worldwide, high-mortality flu pandemic such as the one that occurred after the end of World War I will happen, it’s simply a question of "when."