We identified three primary mechanisms of spillovers : reduced disease transmission , social proximity and substitution of re sources within households .
We found the strongest evidence for spillovers through reduced disease transmission , particularly vaccines and mass drug administration .
In general , the proportion of a population receiving an intervention was associated with improved health .
We found evidence of publication bias for certain spillover estimates but not for total or direct effects .
Conclusions We found the strongest evidence for spillovers from vaccines and mass drug administration to control infectious disease .
There was little high quality evidence of spillovers for other interventions
Background Many interventions delivered to improve health may benefit not only direct recipients but also people in close physical or social proximity .
Our objective was to review all published literature about the spillover effects of interventions on health outcomes in low-middle income countries and to identify methods used in estimating these effects .
Assessment of herd protection against trachoma due to repeated mass antibiotic distributions: a cluster-randomised trial
BACKGROUND Trachoma-control programmes distribute oral azithromycin to treat the ocular strains of chlamydia that cause the disease and to control infection . Theoretically , elimination of infection is feasible if untreated individuals receive an indirect protective effect from living in repeatedly treated communities , which is similar to herd protection in vaccine programmes . We assessed indirect protection against trachoma with mass azithromycin distributions . METHODS In a cluster r and omised trial , 24 subkebeles ( government-defined units ) in Amhara , Ethiopia , were r and omised , with use of a simple r and om sample , to distribution four times per year of single-dose oral azithromycin to children aged 1 - 10 years ( 12 subkebeles , 4764 children ) , or to delayed treatment until  after the  study ( control ; 12 subkebeles , 6014 children ) . We compared the prevalence of  ocular chlamydial infection  in untreated individuals 11 years and older between baseline and 12 months in the treated subkebeles , and at 12 months between the treated and control subkebeles . Health-care and laboratory personnel were blinded to study group . Analysis was intention to treat . The study is registered with clinical trials.gov , number NCT00322972 . FINDINGS At 12 months , 637 children aged 1 - 10 years and 561 adults and children aged 11 years and older were analysed in the children-treated group , and 618 and 550 , respectively , in the control group . The mean prevalence of infection in children decreased from 48.4 % ( 95 % CI 42.9 - 53.9 ) to 3.6 % ( 0.8 - 6.4 ) after four mass treatments . At 12 months , the mean prevalence of infection in the untreated age group ( > /=11 years ) was 47 % ( 95 % CI 33 - 57 ) less than baseline ( p=0.002 ) , and 35 % ( 95 % CI 1 - 57 ) less than that in untreated communities ( p=0.04 ) . INTERPRETATION Frequent treatment of children , who are a core group for transmission of trachoma , could eventually eliminate infection from the entire community . Herd protection is offered by repeated mass antibiotic treatments , providing a strategy for elimination of a bacterial disease when an effective vaccine is unavailable . FUNDING National Institutes of Health
We identified three primary mechanisms of spillovers : reduced disease transmission , social proximity and substitution of re sources within households .
We found the strongest evidence for spillovers through reduced disease transmission , particularly vaccines and mass drug administration .
In general , the proportion of a population receiving an intervention was associated with improved health .
We found evidence of publication bias for certain spillover estimates but not for total or direct effects .
Conclusions We found the strongest evidence for spillovers from vaccines and mass drug administration to control infectious disease .
There was little high quality evidence of spillovers for other interventions
Background Many interventions delivered to improve health may benefit not only direct recipients but also people in close physical or social proximity .
Our objective was to review all published literature about the spillover effects of interventions on health outcomes in low-middle income countries and to identify methods used in estimating these effects .