After premature infants have been vaccinated at around two months of age, they should be careful monitored because there's a heightened risk of heart and respiratory side effects. Researchers from the University of Tennessee Health Science Center, Memphis, studied 239 preterm infants who had being vacci and observed that in a few, cardiorespiratory events were associated with presumed need for intervention." Pourcyrous told Reuters Health that underlying medical conditions and "possibly multiple injections are associated with cardiorespiratory events." At an age of at least 2 months, the infants were given between one and five vaccines simultaneously. Among these were diphtheria-tetanus-pertussis combination vaccines and inactivated polio-virus vaccine. The infants were monitored for three days. Within 48 hours, 16 percent of the infants had had cardiorespiratory difficulties, such as temporary cessation of breathing or a slow heart rate. There was a weak link between such occurrences and having been given multiple injections. The researchers conclude that after vaccination, preterm infants require "close monitoring ... for up to 48 hours." Journal of Pediatrics, August 2007