Frequency of HIV-1 dual subtype infections, including intersubtype superinfections, among injection drug users in Bangkok, Thailand.

Objectives: To estimate the frequency and incidence of dual HIV-1 subtype infections, including superinfections, among recent seroconvertors from a cohort of injection drug users (IDUs). Methods: A total of 1209 HIV-negative IDUs were followed in a prospective cohort study at 15 methadone clinics in Bangkok, Thailand. After 2308 person-years (PY) of follow-up, 133 seroconverted to HIV-1, of which approximately 20% were subtype B and 80% were CRF01―AE (formerly called subtype E). Specimens from 126 individuals were available at time of first seropositive test and specimens from 80 of these 126 individuals were also available more than 12 months later. For each infected participant, we calculated the amount of time to superinfection, loss to follow-up, or to the closest visit more than 12 months after the time of initial seropositivity. Results: Of all 126 seroconverters seen at the time of the first seropositive test result, there was no apparent case of concurrent dual subtype infection detected despite 2301 PY of observation. Overall, the incidence of superinfection was 2.2 per 100 PY [95% confidence interval (Cl), 0.3-7.8]. The 1-year incidence of CRF01―AE superinfection following subtype B primary infection was 3.9 per 100 PY (95% Cl, 0.1-21.9) and the incidence of subtype B superinfection following CRF01―AE primary infection was 1.5 per 100 PY (95% Cl, 0.04-8.3). Conclusions: Determination of the frequency and incidence of dual HIV-1 subtype infection demonstrates that HIV-1 superinfection is not uncommon in a population with high HIV-1 incidence with more than one circulating strain.