I came to post the news about Pfizer working on the variants - but someone beat me to it :).
Catching covid in hospital is more likely to be from infected patients than staff. Staff will be wearing masks, people coming in from the community dont. When community transmission is high some of them are infected but not showing symptoms. You test them if you can but may not get results immediately. Meanwhile if you dont have a single room for them they go on a ward with non covid patients and if they are infected may infect the whole ward before they test positive.
So it's probably the general reduction in infection in the community causing the fall.
NHS staff may still have lateral flow tests to use up even if vaccinated, that would allow the NHS to track infections in vaccinated staff.
It's perfectly possible for NHS staff to still not have caught covid, although large numbers have certainly done so and some have been unlucky enough to have it twice.
Staff will not be vaccinated if pregnant or ttc. Some were not initially allowed vaccination because of severe allergies. I know a lot of NHS staff and the only ones I know not being vaccinated were in one of those groups. However some have only recently been offered a vaccine and that includes people working with covid patients, not all trusts had supplies for staff pre christmas.