reassortment

Reassortment is a genetic mechanism in segmented RNA viruses by which co-infection of a single host cell with two distinct strains leads to progeny virions that carry a mixture of genome segments from both parents.

In influenza A and B viruses, which have eight RNA segments, reassortment can shuffle segments encoding surface antigens such as hemagglutinin (HA) and neuraminidase (NA) or internal proteins involved in replication.

Process occurs when packaging of genome segments into new virions is not strictly tied to the parent origin; during budding, segments from either parent can be incorporated, producing reassortant genomes.

This can create novel antigenic combinations, potentially altering host range, virulence, transmission, or replication efficiency.

It is distinct from genetic recombination within a segment; reassortment exchanges whole segments.

Antigenic shift is a term used to describe major reassortment events that yield new HA and/or NA antigens and can trigger pandemics, whereas drift refers to gradual accumulation of mutations.

Historical examples include the 1957 Asian flu (H2N2) arising from reassortment, the 1968 Hong Kong flu (H3N2) from reassortment, and the 2009 H1N1 pandemic virus, which was a quadruple reassortant with segments of human, swine, and avian origin.

Implications and surveillance: Monitoring reassortment in influenza is crucial for vaccine strain selection and pandemic preparedness.

Limitations: Not all reassortants are viable; compatibility of gene segments is required, and fitness costs can reduce spread.