Influenza, commonly known as "the flu" is a serious, infectious, respiratory illness caused by the influenza virus. The national seasonal flu vaccine is determined each year by the CDC. The quadrivalent flu vaccine now consists of 4 like-virus strains which are termed "killed viruses". These 4 dead strains are predetermined as the most likely to affect the U.S. population the following flu season. Since the virus strains are "killed", it is impossible to catch the flu from a flu vaccination. The flu vaccine offers an approx. 9 month window of protection. Once the vaccine is given, it generally takes up to 2 weeks to reach full immunity from the flu.
It is recommended by the WHO-World Health Organization,
that quadrivalent vaccines for use in the 2020-2021 influenza season, northern hemisphere winter, contain the following:
A/Guangdong-Maonan/Hawaii/2019 (H1N1)pdm09-like virus;
A/Hong Kong/2019 (H3N2)-like virus;
B/Washington/02/2019 (B/Victoria lineage)-like virus; and
B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.