H5N1 influenza strain raises concern about a pandemic
If the virus mutates and can efficiently transmit from person to person, it could cause a deadly pandemic.
by Marie Rosenthal
Editor in Chief, Infectious Disease News
September 2005
WASHINGTON — The human avian H5N1 influenza cases occurring in Asia bring the threat of an influenza pandemic ever closer to our shores, said Benjamin Schwartz, MD, at a press conference sponsored here by the National Foundation for Infectious Diseases.
So far, there has been no confirmed person-to-person transmission, but if the strain mutates and develops that transmission capability, it could cause a deadly pandemic.
The Trust for America’s Health (TFAH) released state-by-state projections that found more than 500,000 Americans could die and more than 2.3 million could be hospitalized if a moderately severe strain of a pandemic influenza virus hit this country. Based on its estimates, 67 million Americans are at risk of contracting the pandemic influenza strain. A pandemic with the H5N1 strain could be worse though, according to Schwartz. The mortality rate so far is higher than 50%.
A possible pandemic
One would be hard pressed to find a public health official or infectious disease specialist who does not believe a pandemic is possible and probable, and that this H5N1 strain increases the likelihood of it being sooner rather than later. “I think [H5N1] has all of us not only fascinated, but anxious. I did not think that it would spread that quickly,” William Schaffner, MD, told Infectious Disease News.
“Influenza pandemics occur when a new influenza strain to which most or all of the population is susceptible spreads among people. This new strain can develop by the sharing of genetic material between avian and human influenza virus strains, either in a coinfected animal such as a pig or in humans, or possibly by mutation of an avian influenza strain,” explained Schwartz, who is senior service advisor, National Vaccine Program Office at the Department of Health and Human Services (HHS), and who is working on the national pandemic plan.
The chance of a recombination depends on the viral strain and how widespread it is among humans and animals. This is why the current H5N1 strain in Asia represents a major pandemic threat. Its scope is unprecedented, Schwartz said. H5N1, which can be asymptomatic, infects domestic poultry and wild waterfowl. Infected migrating birds can carry the virus long distances as well.
People with H5N1 infection generally have severe disease, Schwartz said. “Unlike the flu that occurs in the United States each year, which most often causes mild disease, reported H5N1 cases usually progress to pneumonia, frequently progress to respiratory failure and accompanying multiorgan system failure and, in more than half of all cases, death occurs.”
A normal flu season tends to kill the most vulnerable populations: the elderly, infants and those with chronic conditions. Most of the H5N1 deaths have occurred in young and previously healthy individuals, Schwartz added.
The ongoing disease in domestic and wild poultry and the evolution of the H5N1 virus, which appears to be adapting to spread among mammals, point to a growing threat. “However, it is unclear whether this strain of H5N1 will ever acquire the ability for efficient and sustained transmission among people,” he said.
So, public health officials prepare for a pandemic with an eye on H5N1, but a realization that the threat could be lurking somewhere else. The influenza pandemic of 1918 probably started in the middle of the United States, although it is difficult to pinpoint exactly where.
Many steps must be taken to prepare for a pandemic. Surveillance must be enhanced. The production capacity of influenza vaccines and antiviral drugs must be increased, and the capabilities of the health care systems to respond must be improved to reduce death and severe illness among those who contract the disease.
“Early detection of influenza among animals and people is critical,” Schwartz said, but many countries affected by avian influenza lack good surveillance systems, as well as the necessary laboratory, epidemiologic and veterinarian staff.
The NIH and WHO have provided money and manpower to improve the surveillance infrastructure of Asian countries, to strengthen detection and response capabilities, and to improve data management and information sharing.
From detection to protection
Vaccination will be the primary prevention strategy in a pandemic. “However, we face substantial challenges in developing and producing pandemic vaccine. Developing a vaccine to a new influenza strain may take up to six months,” Schwartz said, which makes early detection even more critical.
Even if there is a vaccine, the need will exceed production capacity. “Unlike annual influenza, where we focus our efforts on only part of the population, in a pandemic, everyone will be susceptible and vaccination most likely will be recommended universally,” he said. “And because every country, not just the United States, will be experiencing the pandemic, we assume that only vaccine manufactured within the United States will be available for our population, despite preexisting contractual agreements.”