A quick Googling shows that about 88% of Portuguese were fully vaccinated just as Omicron was arriving. And in the article they talk about it being much higher by the time BA.5 arrives.
There could be two reasons that I can think of for the improved immunity.
Either there is a recency effect, which should be washed out a bit with boosters. Like, if you got the original strain but then got a booster just before BA.5 or if Omicron acted like your booster (since they were already probably vaccinated), you would expect both cohorts to have increased resistance.
The second is that Omicron is "better designed' than the boosters for fighting off BA.5. And I think that would say something interesting about antigenic sin, where getting an obsolete vaccine first didn't sufficiently reduce the ability of Omicron to give you a booster to erode the signal.
There could be two reasons that I can think of for the improved immunity.
Either there is a recency effect, which should be washed out a bit with boosters. Like, if you got the original strain but then got a booster just before BA.5 or if Omicron acted like your booster (since they were already probably vaccinated), you would expect both cohorts to have increased resistance.
The second is that Omicron is "better designed' than the boosters for fighting off BA.5. And I think that would say something interesting about antigenic sin, where getting an obsolete vaccine first didn't sufficiently reduce the ability of Omicron to give you a booster to erode the signal.