The UK is monitoring reinfections in a group of 9813 health care workers with prior infection. In order to count as a potential reinfection, they must have a second positive PCR test more than 90 days after the first. This population is having routine PCR screening every two weeks, which allows for the discovery of asymptomatic infections.
The winter increase in cases overall brought an increase in reinfections to this population. However, in late Dec / early Jan this population became eligible for vaccination (due to being health care workers). People with prior infection have a very strong antibody response to vaccination, and 95% of this population elected to get the vaccine. It appears that those strong reactions paid off, because reinfections plummeted after vaccines were offered.
I'd love to know if those few people who had infections in March-May were in the 5% who were not vaccinated, or if they were breakthrough cases in those with vaccine on top of prior infection. But alas, we don't have that data.
Link to source : https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/991343/Variants_of_Concern_VOC_Technical_Briefing_14.pdf