The National Institutes of Health announced Tuesday it has started an early stage clinical trial looking at what happens when an adult who is fully vaccinated with one type of Covid-19 vaccine, like Pfizer’s, is boosted with a different shot about three to four months later. The trial will include about 150 adults who have received one of the three Covid vaccine regimens currently available under the Food and Drug Administration’s emergency use authorization: Johnson & Johnson’s, Moderna’s or Pfizer’s. Federal health officials said people who have not yet received an authorized vaccine are also eligible to enroll in the trial in a separate group. Those volunteers will receive two doses of Moderna’s vaccine and will be assigned to receive a booster dose of one of the three vaccines about 12 to 20 weeks later, officials said.