related_tweet: @shesallthatnz @alannah_27 @nzstuff the seasonal flu is the most obvious, and covid acts similar. after 6 months the flu shot is ineffective as it wanes, and also the virus mutates within that time to avoid it. many others last 10 or less years. hpv, hib, diphtheria. some even less eg. pertussis, and pneumococcal.