Influenza Virus Characteristics and Transmission

Introduction to Influenza Virus Types

Influenza A and B are the two types of influenza viruses that cause epidemic human disease, with influenza A viruses further categorized into subtypes based on two critical surface antigens: hemagglutinin (H) and neuraminidase (N) 1, 2, 3
The Centers for Disease Control and Prevention (CDC) notes that influenza B viruses are not categorized into subtypes and undergo antigenic drift less rapidly than influenza A viruses 1, 2, 3
Since 1977, influenza A (H1N1), influenza A (H3N2), and influenza B viruses have been in global circulation 1, 2, 3
Influenza A (H1N2) viruses emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses and have been detected in many countries 1, 2, 3

The World Health Organization (WHO) states that influenza viruses undergo two distinct mechanisms of antigenic change: antigenic drift through point mutations during viral replication, and antigenic shift involving major changes through genetic reassortment between different strains 1, 2, 3, 4
Antigenic drift results from frequent point mutations and recombination events during viral replication, creating new influenza virus variants continuously 1, 2, 5
Influenza A viruses undergo antigenic drift more rapidly than influenza B viruses, which is the virologic basis for seasonal epidemics and necessitates annual vaccine strain updates 1, 2, 6
Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype, and antibody to one antigenic variant might not protect against a new antigenic variant of the same type or subtype 1, 2, 6

The CDC recommends that influenza viruses spread from person to person primarily through respiratory droplets from coughing and sneezing of infected persons 1, 2, 3, 4
The incubation period ranges from 1-4 days, with an average of 2 days 1, 2, 3
Adults are typically infectious from the day before symptoms begin through approximately 5 days after illness onset 1, 2
Children can be infectious for longer periods, up to 10 days or more 2, 3
Severely immunocompromised persons can shed virus for weeks or months 2, 3