Dose-Ranging Study to Evaluate a 25-Valent Pneumococcal Conjugate Vaccine

Inclusion Criteria: * Healthy adults who are 18 through 49 years old on the day of randomization (Day 1). * Participant must provide voluntary written informed consent to participate in the study. * Participant must be able to comprehend and comply with study requirements and procedures and be willing and able to return for all scheduled follow-up visits. * Adult female participants who are not surgically sterile must have a negative pregnancy test at screening and negative pregnancy test prior to vaccination and must agree to employ a highly effective method to avoid pregnancy through Day 57 of the study. Exclusion Criteria: * Use of any investigational medicinal product within 90 days prior to randomization or planned use of such a product during the period of study participation. * Adults who have previously been vaccinated against S. pneumoniae. * History of microbiologically confirmed invasive disease caused by S. pneumoniae. * History of allergic disease (including angioedema) or history of a serious reaction to any prior vaccination or known hypersensitivity to any component of the study vaccines, including PEG. * Any abnormal vital sign deemed clinically relevant by the PI. * Acute illness at time of randomization (moderate or severe) and/or fever (body temperature of ≥ 38.0°C) * History of any non-study vaccine administration within 14 days of study vaccine administration. * No planned vaccines until after Day 29 (Visit 3). * Chronic administration (defined as more than 14 consecutive days) of immunosuppressant or other immune modifying drugs prior to the administration of the study vaccine (and within the 6 months prior to administration of the study vaccine), including the use of glucocorticoids. The use of topical and inhaled glucocorticoids will be permitted. * Administration of immunoglobulins and/or any blood products within the 6 months prior to administration of the study vaccine or anticipation of such administration during the study period. * Any medical or social condition that in the opinion of the PI , may interfere with the study objectives, pose a risk to the participant, or prevent the participant from completing the study follow-up. * Any screening laboratory test result outside the normal range and with toxicity score ≥ 2, unless allowed by study team. * A positive serologic test for human immunodeficiency virus (HIV)-1 or HIV-2 (HIV 1/2 Ab), hepatitis B (HBsAg) or hepatitis C (HCV Ab). * History of malignancy, excluding non-melanoma skin and cervical carcinoma in situ. * Recent history (within the past year) or signs of alcohol or substance abuse. * History of major psychiatric disorder. * Female adult participants who are pregnant or breastfeeding. * Participant is an employee of, or direct descendant (child or grandchild) of any person employed by the Sponsor, PATH, the Contract Research Organization (CRO), the PI.