Investigators have made rapid progress on several fronts in understanding the novel influenza A(H1N1) virus responsible for the current pandemic, according to a series of reports.
Investigators tracing the virus's evolutionary history have learned that the novel H1N1 flu virus is a direct, fourth-generation descendant of the virus that caused the disastrous 1918 flu pandemic. They've learned that the most common rapid influenza test is highly specific but not very sensitive for detecting the influenza A(H1N1) virus.
In addition, the virus's worldwide spread closely matched historic patterns of airline travel from Mexico. Furthermore, the unusual age distribution of severe illness and death associated with the virus, affecting mostly children and young adults, may have resulted from relative protection among older individuals who had been exposed to H1N1 during childhood before the 1957 pandemic.
All of the reports are available online at the New England Journal of Medicine's online H1N1 influenza center (http://h1n1.nejm.org
A review article, by Dr. Shanta M. Zimmer and Dr. Donald S. Burke of the University of Pittsburgh, traced the emergence of the influenza A(H1N1) virus to an avian virus that simultaneously appeared in humans and swine in 1918. That virus killed 40 million to 50 million people.
The influenza A(H1N1) virus consists of eight genes that have mutated steadily between 1918 and the present, but the virus has acquired no new gene segments from avian or other sources.
The virus disappeared entirely from humans in 1957 and was replaced by a new strain called H2N2. H1N1 was not detected again until 1976, when the virus was transmitted from a swine to humans, causing an outbreak of respiratory disease among soldiers at Fort Dix, N.J.
Beginning in 1977, the H1N1 virus, accompanied by H3N2, began to co-circulate seasonally in humans. Since then, new H1N1 strains have emerged in swine, with occasional cross-species transmission to humans. In 2008, two distinct H1 swine viruses combined to produce the virus causing the current pandemic.
In an editorial, Dr. David M. Morens, Dr. Jeffrey K. Taubenberger, and Dr. Anthony S. Fauci of the National Institute of Allergy and Infectious Diseases likened the interaction between the various descendants of the 1918 virus and the human community as an “elaborate dance.” They wrote that the “partners have remained linked and in step, even as each strives to take the lead,” and that there's little evidence that this era is about to come to an end.
In a letter to the editor, Dr. Dennis J. Faix of the Naval Health Research Center in San Diego and his colleagues determined that the most commonly used rapid influenza test, called QuickVue Influenza A+B, widely available in doctors' offices in the United States, has a 99% specificity for the novel H1N1 flu virus, compared with definitive polymerase chain reaction-based tests. This means that if the test indicates that the patient's virus is H1N1, it almost certainly is. On the other hand, the sensitivity of the test ranged from 31% to 63% in various populations, meaning that the test may well miss genuine cases of novel H1N1 flu.
In another letter to the editor, Dr. Kamran Khan of St. Michael's Hospital in Toronto and colleagues found that the March through April 2009 worldwide spread of the virus closely matched patterns of global airline transportation originating from Mexico during the same period in 2008. Of 20 countries with the highest volumes of international passengers arriving from Mexico, 16 had confirmed importations of H1N1. (The exceptions were Japan, Chile, Venezuela, and Peru.)
They calculated that countries receiving more than 1,400 passengers from Mexico were at significantly higher risk of importation, and that international air traffic volume alone was 92% sensitive and more than 92% specific in predicting importation.
In an original research study, Dr. Rogelio Pérez-Padilla of the National Institute of Respiratory Diseases of Mexico in Mexico City and colleagues investigated 18 patients who were hospitalized with pneumonia between March 24 and April 24, 2009, and confirmed novel H1N1 flu.
More than half of those patients were between ages 13 and 47 years, and only eight had preexisting medical conditions. Twelve patients required mechanical ventilation, and 7 died. Within 7 days after contact with these initial cases, 22 health care workers developed mild or moderate flulike illness, but none required hospitalization.
In a much larger study involving 2,155 cases of severe pneumonia reported to the Mexican Ministry of Health, Gerardo Chowell, Ph.D., of the National Institutes of Health, Bethesda, Md., and colleagues found that 87% of the deaths and 71% of the cases of severe pneumonia involved patients between ages 5 and 59 years.