BREAKTHROUGH INFECTION; SUSPECTED CLINICAL VACCINATION FAILURE; This spontaneous report received from a health care professional via other pharmaceutical company (Pfizer) concerned multiple patients. No past medical history or concurrent conditions were reported. Patients received covid-19 vaccine ad26.cov2.s (suspension for injection, route of admin, and batch number were not reported) dose, start therapy date were not reported, 1 total administered for prophylactic vaccination. The batch numbers were not reported. The company is unable to perform follow-up to request batch numbers. No concomitant medications were reported. On an unspecified date, the patients experienced  a breakthrough infection, and suspected clinical vaccination failure. As per the reporter, "The protective power of Johnson & Johnson's single-dose vaccine plunged from 86 percent to just 13 percent over those six months (01-FEB-2021 to 01-OCT-2021). The effectiveness of shots made by Pfizer and BioNTech, which also employed two doses, fell from 87% to 45% in the same period. When older veterans who got a single jab of the J&J vaccine suffered a breakthrough infection they were 52% less likely to die than their peers who didn't get any shots and when younger veterans inoculated with the J&J vaccine suffered a breakthrough infection, they were 73% less likely to die of COVID-19 than were their unvaccinated peers. The Centers for Disease Control and Prevention has recommended booster shots for everyone who got the Johnson & Johnson vaccine at least two months earlier. The boosters were also recommended six months after a second dose of the Moderna or Pfizer vaccines for everyone 65 and older; those with medical conditions that make them more vulnerable to a serious case of COVID-19; those who live in nursing homes or other group settings; and those who live or work in high-risk settings such as hospitals or prisons. By the end of September, Moderna's two-dose COVID-19 vaccine, measured as 89% effective in March, was only 58% effective. In addition, all people with compromised immune systems are advised to get a booster shot if it's been at least 28 days since their vaccine took full effect. As per the reporter the study tracked 780,225 veterans of the U.S. armed forces from Feb. 1 to Oct. 1. Close to 500,000 of them had been vaccinated, while just under 300,000 had not." The action taken with covid-19 vaccine ad26.cov2.s was not applicable. The outcome of the breakthrough infection and suspected clinical vaccination failure was not reported. This report was serious (Other Medically Important Condition). This report was associated with regulatory authority.; Sender's Comments: V0: 20211140408-COVID-19 VACCINE AD26.COV2.S-suspected clinical vaccination failure. This event(s) is considered not related. The event(s) has a compatible/suggestive temporal relationship, is unlabeled, and has unknown scientific plausibility. There are other factors more likely to be associated with the event(s) than the drug. Specifically: SPECIAL SITUATIONS 20211140408-COVID-19 VACCINE AD26.COV2.S- breakthrough infection. This event(s) is considered unassessable. The event(s) has a compatible/suggestive temporal relationship, is unlabeled, and has unknown scientific plausibility. There is no information on any other factors potentially associated with the event(s).