both ears) are linked to the administration of covid‑19 vaccine janssen." tinnitus was later labeled as "very rare" in a final safety study by the manufacturer. in december 2021, the cdc accepted the recommendation from a panel of experts for a preference of using the pfizer-bionech and moderna vaccines over the janssen vaccine due to rare but serious blood clotting events. in may 2022, the fda limited the use of the janssen vaccine to those over eighteen unable to access other vaccines or who are otherwise "medically ineligible" for other vaccine options. history the stabilized version of the spike protein – that includes two mutations in which the regular amino acids are replaced with prolines – was developed by researchers at the national institute of allergy and infectious diseases' vaccine research center and the university of texas at austin. during the covid‑19 pandemic, johnson & johnson committed over us$1 billion toward development of a not-for-profit vaccine in partnership with the biomedical advanced research and development authority (barda) office of the assistant secretary for preparedness and response (aspr) at the u.s. department of health and human services (hhs). johnson & johnson said its vaccine project would be "at a not-for-profit level" as the company viewed it as "the fastest and the best way to find all the collaborations in the world to make this happen". in november, johnson & johnson announced that janssen would commit about $604 million and barda would commit $454 million to fund the ensemble trial. inside an emergent biosolutions facility where, in collaboration with johnson & johnson, vaccines are produced johnson & johnson subsidiary janssen vaccines, in partnership with beth israel deaconess medical center (bidmc), was responsible for developing the vaccine candidate, based on the same technology used to make its ebola vaccine. clinical trials preclinical trials indicated that the vaccine effectively protected hamsters and rhesus macaques from sars‐cov‐2. phase i–ii in june 2020, johnson & johnson and the national institute of allergy and infectious diseases (niaid) confirmed that they planned to start clinical trials of the ad26.cov2.s vaccine in september 2020, with the possibility of phase i–iia human clinical trials starting at an accelerated pace in the second half of july. a phase i–iia clinical trial started with the recruitment of the first subject on 15 july 2020 and enrolled study participants in belgium and the us. interim results from the phase i–iia trial established the safety, reactogenicity, and immunogenicity of ad26.cov2.s. with one dose, after 29 days, the vaccine ensured ninety percent of participants had enough antibodies required to neutralize the virus. after 57 days, that number reached one hundred percent. 1x1011 viral particles (high dose) provided an increase in the neutralizing-antibody titers compared to 5×1010 (low dose). after the second dose 56 days after the first dose among participants between the ages of 18 and 55 years, the incidence of grade 3 solicited systemic adverse events was much lower than that after the first immunization in both the low-dose and high-dose groups, a finding that contrasts with observations with respect to messenger rna–based vaccines, for which the second dose has been associated with increased reactogenicity. a substudy with 20 participants found that humoral and cell-mediated immune responses, including cytotoxic t cells, lasted for at least 8 months. phase iii a phase iii clinical trial called ensemble started enrollment in september 2020 and completed enrollment on 17 december 2020. it was designed as a randomized, double-blind, placebo-controlled clinical trial intended to evaluate the safety and efficacy of a single-dose vaccine versus placebo in adults aged 18 years of age and older. study participants received a single intramuscular injection of ad26.cov2.s at a dose level of 5×1010 virus particles on day one. the trial was paused on 12 october 2020, because a volunteer became ill, but the company said it found no evidence that the vaccine had caused the illness and announced on 23 october 2020 that it would resume the trial. on 29 january 2021, janssen announced safety and efficacy data from an interim analysis of ensemble trial data, which demonstrated the vaccine was 66% effective at preventing the combined endpoints of moderate and severe covid‑19 at 28 days post-vaccination among all volunteers. the interim analysis was based on 468 cases of symptomatic covid‑19 among 43,783 adult volunteers in argentina, brazil, chile, colombia, mexico, peru, south africa, and the united states. no deaths related to covid‑19 were reported in the vaccine group, while five deaths in the placebo group were related to covid‑19. during the trial, no anaphylaxis was observed in participants. a second phase iii clinical trial called ensemble 2 started enrollment on 12 november 2020. ensemble 2 differed from ensemble in that its study participants received two intramuscular (im) injections of ad26.cov2.s, one on day 1 and the next on day 57. early results indicated 85% efficacy against severe/critical disease. plasma from 8 participants showed greater neutralization activity against the delta variant than against beta. authorizations full authorization full authorization, not used emergency authorization allowed for travel eligible covax recipient usage stopped