Author summary Why was this study done? There is little information in the current literature on pneumococcal transmission to infants. Since individuals in this age group are at major risk of pneumococcal disease and at potentially reduced protection in reduced dose pneumococcal vaccination schedules, we carried out a study to identify their close contacts and likely routes of transmission. What did the researchers do and find? We identified all 7,428 physical contacts of 1,583 infants in Nha Trang, a coastal city in Vietnam. We found that there was a high correlation between infant infection and their probability of contact with at least 1 pneumococcal carrier on the surveyed day. Preschool-age children contributed around half of the total direct pneumococcal exposure risk to infants in Nha Trang. Around 80% of the exposure risk came from contact with household members. What do these findings mean? Social contact surveys can help to identify possible transmission pathways. Infants in Nha Trang are likely to receive substantial indirect protection against pneumococcal infection from vaccinated older siblings.