Influenza pandemics result from Influenza A undergoing antigenic shift
, creating a new virus capable of being transmitted between humans.
Comparison of influenza viruses (a) Characteristic Influenza A Influenza B Genome Negative-strand RNA, Negative-strand RNA, eight segmented genes eight segmented genes Sequence High Moderate variability in HA and NA Mode of antigen Antigenic drift and Antigenic drift only variation antigenic shift
Hosts Humans, avian, swine, Humans only equine, marine mammals Epidemiology Epidemic, pandemic Epidemic, nonpandemic (a) RNA, ribonucleic acid; HA, hemagglutinin; NA, neuraminadase.
Here's where the antigenic shift
comes in: If chromosomes from the two viruses recombine within infected cells in just the right formula, they can spawn a new virus with antigens that the human body may not recognize.
and increased incidence of meningococcal disease.
Conditions favorable for the emergence of antigenic shift
have long been thought to involve humans living in close proximity to domestic poultry and pigs.
The concept is based on identifying alternative influenza antigens that are not as susceptible to antigenic shift
Governments are now preparing contingency plans against the effects of an expected further antigenic shift
A more drastic antigenic change, termed antigenic shift
, occurs if a new HA subtype is introduced into the pool of human virus strains by reassortment of genes between animal and human strains or by direct transmission of strains from an animal reservoir to humans, as has occurred recently with strains of H5N1, H7N7, and H9N2 (1).
The need for a new vaccine was apparent (7), but early reports consistently described the disease as mild (8), and the US epidemic was over before the A2/Hong Kong virus was recognized as an antigenic shift
Francis died in 1969 and did not live to know the full explanations for antigenic shift
through reassortment of gene segments from 2 parent viruses or antigenic drift through mutation.
In the 20th century, the sudden emergence of autigenically different strains transmissible in humans, termed antigenic shift
, has occurred on four occasions, each time resulting in a pandemic.
Influenza vaccine--outmaneuvering antigenic shift
Similarly, pandemic strains of influenza developed through an antigenic shift
from avian influenza A viruses.