Effectiveness of monovalent rotavirus vaccine (rotarix) against severe diarrhea caused by serotypically unrelated G2P[4] strains in Brazil

Background. In a Latin American trial, a monovalent G1P[8] rotavirus vaccine showed high efficacy against severe rotavirus diarrhea. Protection was lower against serotypically unrelated G2P[4] strains, which circulated infrequently. This case-control study was undertaken to assess the effectiveness of this monovalent G1P[8] rotavirus vaccine against G2P[4] strains in Brazil. Methods. Case patients were children with severe G2P[4] rotavirus diarrhea who presented at a hospital in Recife, Brazil, from March 2006 through September 2008. Vaccination rates among case patients were compared with rates among 2 groups of control participants - children with rotavirus-negative diarrhea and children admitted for acute respiratory tract infection (ARI) - to calculate vaccine effectiveness, after controlling for the birth month and year. Results. We enrolled 70 G2P[4] rotavirus-positive case patients with severe diarrhea, 484 rotavirus-negative control participants with diarrhea, and 416 control participants with ARI, aged ≥6 months. Among children aged 6-11 months, the effectiveness of the vaccine against G2P[4] diarrhea was 77% (95% confidence interval [CI], 42%-91%) and 77% (95% CI, 43%-90%) among the rotavirus-negative control participants with diarrhea and control participants with ARI, respectively. Vaccine effectiveness in children aged ≥12 months decreased to -24% (95% CI, -190% to 47%) and 15% (95% CI, -101 to 64) among the rotavirus-negative control groups with diarrhea and ARI, respectively. Conclusions. This monovalent G1P[8] rotavirus vaccine was effective against severe G2P[4] rotavirus diarrhea among children aged 6-11 months. Effectiveness declined among children aged ≥12 months, which suggests waning immunity.