*Sexual activity was defined as direct contact of a sexual nature, such as kissing, oral sex, and penetrative (vaginal, anal) sex. Some categories in this table were collapsed to avoid deductive disclosure. Median (interquartile range) age, 40 (32–43) y. PrEP, pre-exposure prophylaxis; STI, sexually transmitted infection. †Sex-on-premises venues, private sex parties and cruising grounds are activities that took place either in the United Kingdom or abroad. ‡New partner, person with whom the index case-patient is likely to have had sex on >1 occasion; one-time partner, person with whom the index case-patient has had sex on 1 occasion only; occasional partner, person with whom the index case-patient has had sex on >1 occasion and with whom there is an expectation of sex again on a sporadic or regular basis; established partner, primary partner or secondary partner (e.g., a long-term affair) (10); chemsex, use of drugs such as GHB (gamma-hydroxybutyrate), crystal methamphetamine, or mephedrone during sex.