She received 2 doses? After one dose you can still get infected. Even after the full vaccine, they say you can still get the virus it's just that you won't get sick from it but you can still pass it onto someone else.
Another possibility is that she might have gotten the South African strain for which the pfizer vaccine is slightly less effective.
Still better than any Chinese ones though. All of the adenovirus-type vaccines are useless against the mutated strains.
Shouldn't be that much of SA variant around, who knows, only single dose, only 1 case so far, contracted before 3 weeks it takes to kick in aswell.
Over 70s have had there vaccine 1st dose only for a while and that age group isn't coming down faster than other age groups hospital admissions wise, see if deaths drop off soon but lagging, so looks like not working so far.
A lot of hospitalisation and deaths in the 70+ from the Oxford Astro vaccine aswell, it's side effects are too much for many to handle sadly, likely a lot of the deaths the last 2 months in the UK but all going down as covid deaths, people realising but to late

. Screwing up the data.
It'll keep mutating and they'll keep rushing and botching vaccines and pumping into us until 1 goes wrong and kills / screws us up badly,. Mutations only started with the vaccines, vaccines giving them the advantage to become dominant rather than just dying out, so chasing our tails out of fear here.
Still better off ignoring Covid damage being done is crazy and none of the measures are actually working, most people I know snapping point some lockdowns some fear of virus, few already snapped, wish I lived in Sweden where ignoring it is working better than UK.
We'll never get back holidays abroad, this isn't about a virus any more, it's mutating into green leftie communist agenda, of total control and zero fun ever again
