The phrase "subsequent boosters" refers to additional doses of a vaccine given after the initial doses. These extra shots help to strengthen and prolong the protection provided by the vaccine. Full definition
Research has shown a high rate of success in the treatment and prevention of Canine Parvovirus using this homeopathic «vaccine» in contrast to the conventional vaccination which seldom confers immunity without numerous subsequent booster shots. (dog-health-guide.org)
If your dog is at least six weeks old, he should be given the parvo shot with subsequent boosters three weeks and six weeks after the initial vaccine. (dogs.lovetoknow.com)
Volunteers from the trial will now be examined for immune responses — particularly neutralizing antibodies as well as cellular immunity in T cells — and some will get subsequent booster shots to see if protection can be sustained. (scientificamerican.com)