All flu vaccines administered in the U.S. are quadrivalent vaccines. They provide protection against four unique flu viruses: an influenza A(H1N1) virus, an influenza A(H3N2) virus, and two influenza B viruses.
The intravenous flu shot and the nasal spray flu vaccine are both typically produced by growing candidate vaccine viruses in fertilized hens' eggs and then purifying the viral antigen—or the virus particles that help the immune system mount its response to the virus. A flu vaccine can also be made by growing candidate viruses in mammalian cells and then purifying the viral antigen. The intravenous flu shot is an inactivated (or "killed") vaccine, containing dead strains of the viruses, while the nasal spray is a live attenuated vaccine, meaning the flu strains are alive but in weakened doses.
The egg-based process has been relied on by antivirus developers for more than 70 years, whereas the cell-based process was only approved by the Food and Drug Administration in 2012. The advantage of the mammalian cell-based process is that it is much swifter than the egg-based process and is not contingent on there being enough eggs available for mass production.
A third development process, which uses recombinant technology, was approved by the FDA in 2013. Recombinant flu vaccines are manufactured without hens' eggs or mammalian cells and do not require candidate viruses. To produce recombinant flu vaccines, scientists first isolate the gene containing instructions for making the protein hemagglutinin, which is found on the surface of a flu virus and helps the immune system produce antibodies against the virus. Scientists insert this gene into a baculovirus, a virus that infects invertebrates. This baculovirus transports the instructional gene into a host cell line and instructs the host cells to produce hemagglutinin. The hemagglutinin is then collected, purified, and made into a recombinant flu vaccine.
For the 2022-2023 flu season, while the CDC makes no specific recommendation for which flu shot persons under the age of 65 should seek, it does recommend three different high-dose vaccines as best for those over 65.