Flu vaccine had four strains out of nearly one-hundred monitored worldwide. If you choose poorly upon what’s in the mix, it’s not a successful year for the vaccine. If a virus mutates, the vaccine loses effect.
Yes, and flu is a segmented RNA virus with a high rate of drift, shift, and mutation. Pigs are the mixing vessel for this virus that jumps between birds, pigs, and humans. When pig cells become coinfected with more than one strain of flu, the potential for these changes to result in a pandemic flu is of great concern. Maybe you are effectively vaccinated or maybe you aren’t but it is certain the herd will be culled. Hence, the war against the microbial world continues as it has for thousands of years.