Bill Tindall, E.Tn.
We get colds and flu more than once because the virus that causes it mutates so that our immune system doesn't recognize the virus. How long the immunity lasts is driven by decay of immunity and whether the virus mutates enough that the immune system no longer recognizes it.

There is a credible hypotheses that there is immunity in the population to this virus as a result of prior infection with a similar virus.

Trial participants will be monitored to answer the question of how long vaccine immunity lasts.