Improved methods for the production of reassortant influenza viruses are provided.

The present invention provides high growth influenza reassortant virus and high growth influenza reassortant virus vectors comprising amino acid modifications in the PB2, PB1, M1 and/or NS2 proteins which exhibit highly increased growth rates compared to unmodified influenza virus. Further provided are pharmaceutical compositions comprising reassortant virus and viral vectors comprising said modifications and their use for vaccination purposes.

The invention provides a composition useful to prepare high titer influenza B viruses, e.g., in the absence of helper virus, which includes internal genes from an influenza B virus vaccine strain or isolate, e.g., one that is safe in humans, for instance, one that does not result in significant disease, that confer enhanced growth in cells in culture, such as MDCK cells, or in eggs.

Disclosed herein are compositions and methods related to mutant viruses, and in particular, mutant influenza viruses. The mutant viruses disclosed herein include a mutant BM2 sequence, and are useful in immunogenic compositions, e.g., as vaccines. Also disclosed herein are methods, compositions and cells for propagating the viral mutants, and methods, devices and compositions related to vaccination.

The invention provides a composition useful to prepare high titer influenza B viruses, e.g., in the absence of helper virus, which includes internal genes from an influenza B virus vaccine strain or isolate, e.g., one that is safe in humans, for instance, one that does not result in significant disease, that confer enhanced growth in cells in culture, such as MDCK cells, or in eggs.

Provided herein is a method of functionalizing a particle, as well as methods of optically tracking a particle, isolating enveloped viral particles from a sample, quantifying enveloped virus particles in a sample and assessing enveloped viral aggregation in a sample. Kits are also provided. The particle is typically a viral particle.