#breakthrough

At the risk of provoking Wylted's anti-vax boner, I do see a trend developing that does not comport with my understanding of vaccine effectiveness and post-vaccination breakthrough risks.

I'm talking about recent stories like this:

None of this refutes the conventional wisdom- that vaccines are not 100% effective; that breakthrough cases in the vaccinated are rare; that vaccinated people get less sick than unvaccinated.  However, we are told that the variability in vaccine immunity is individual- that sometimes some immune systems just don't respond as vigorously as we'd hope.  If that is the primary variable however, then the rarity and randomness of breakthrough cases should prevent clusters of cases, right?

So out of a relatively small sampling of Texas Democrats who flew to Washington- at least 3 out of 65 vaccinated all get sick at once.  A 4.6% breakout rate.  On a 40 man Yankee's roster we've had 15 breakthrough cases since the beginning of May- a 37.5% breakthrough rate.

I suppose we don't have enough information yet, and some totally non virological explanation might yet account for these anecdotes (Everybody on the Yankee's lying about getting vaccinated seems possible tho not likely).  I'm just wondering if these breakthrough clusters might not be challenging our understanding of vaccine effectiveness.   Is there some virologic element I'm not considering?   Is plane travel amplifying exposure?  Are these breakthoughs associated with the Delta variant?