how many patients had a swab test last year?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'swab' and datetime(microbiologyevents.charttime,'start of year') = datetime(current_time,'start of year','-1 year') )