Pneumococcal nasopharyngeal carriage in children 2-60 months of age in Novi Sad (Serbia), October 2022-September 2026

Inside the human nose and throat, a quiet ecosystem exists where a common bacterium called Streptococcus pneumoniae often lives without causing any trouble. This state of silent presence is known as carriage. While the bacteria are merely hitching a ride in the upper airways, they are not harmless; this colonization is the essential first step that allows the germ to spread from person to person and, in some cases, to invade deeper tissues to cause serious illnesses like pneumonia, meningitis, or blood infections. Young children are the primary reservoir for this bacteria, carrying it in their noses far more often than adults do. Because these children interact closely with one another, they act as the main engine for transmission within a community. To stop the spread of disease, scientists and doctors rely on vaccines that target specific versions, or serotypes, of the bacteria. However, the bacterial population is not static; when vaccines remove certain types, other types can move in to fill the empty space, a shift known as serotype replacement. Understanding which versions are currently circulating, how often they are carried, and whether they are becoming resistant to antibiotics is critical for keeping vaccination strategies effective and protecting public health.

In the city of Novi Sad, Serbia, a team of researchers set out to map this invisible landscape over a four-year period, from October 2022 to September 2026. They focused on children between the ages of two months and five years, a group that is most vulnerable to both carrying the bacteria and developing disease. The study was designed to capture a realistic picture of the community by recruiting children during two distinct scenarios: those visiting the doctor for routine, healthy check-ups and those seeking care for acute respiratory infections like coughs or fevers. By swabbing the noses of thousands of children, the team collected a vast library of bacterial samples to analyze what was actually present in the community. They did not just count how many children were carrying the bacteria; they identified the specific genetic types of the bacteria found and tested them to see if they could be killed by common antibiotics. This approach allowed them to see not only the current state of the bacteria but also how it was changing over time as the country's vaccination program shifted from one type of vaccine to another.

The results revealed that the bacteria are a common resident in the noses of young children in this region. Out of more than 3,300 children tested, roughly 27 percent were found to be carrying the bacteria. This prevalence was not uniform; it was significantly higher among children who were already sick with a respiratory infection compared to those who were healthy. The data also showed that the likelihood of carrying the bacteria depended heavily on age, with the highest rates found in children between two and three years old. Daycare attendance was another major factor, with children who spent time in group care settings carrying the bacteria much more often than those who did not. The study also captured the rhythm of the seasons, showing that carriage rates peaked during the autumn and winter months and dropped to their lowest point during the summer, a pattern that aligns with the increased indoor crowding and viral circulation typical of colder weather.

A central goal of the research was to see how well the current and future vaccines matched the bacteria actually circulating in the community. The researchers found a diverse mix of bacterial types, with no single version dominating the scene. When they compared these circulating types against the vaccines currently licensed for use, they discovered a significant gap. The most widely used vaccine at the time, which covers ten types of bacteria, matched only about 8 percent of the isolates found. A more comprehensive vaccine covering thirteen types matched about 17 percent, and even a newer, twenty-valent vaccine covered less than half of the bacteria present. This means that more than half of the pneumococcal bacteria circulating in these children belonged to types that are not currently protected against by the standard vaccines. The study also looked at several experimental, higher-valency vaccines that are being developed to cover more types, finding that these future options would theoretically match a much larger portion of the circulating bacteria, though their real-world effectiveness remains to be seen.

Beyond the types of bacteria, the researchers were concerned with how these germs respond to medicine. They tested the bacteria against a range of antibiotics to see if they had developed resistance. The findings showed that resistance to certain drugs, particularly erythromycin, clindamycin, and tetracycline, was quite common, affecting a significant portion of the bacteria. However, resistance to penicillin, a cornerstone antibiotic for treating bacterial infections, remained rare. The study also tracked how these resistance patterns changed over the four years. While resistance to some drugs decreased over time, resistance to another common antibiotic, trimethoprim-sulfamethoxazole, rose sharply in the final year of the study. This fluctuation suggests that the bacterial population is dynamic, constantly shifting in response to environmental pressures.

The study concludes that the landscape of pneumococcal carriage in Novi Sad is complex and constantly evolving. The bacteria are widespread, particularly among young children and those in group settings, and the types circulating are increasingly those not covered by current vaccines. The high rate of carriage in sick children confirms that the nose is a key site for transmission, especially during respiratory illness. While the vaccines have likely reduced the presence of the specific types they target, the bacteria have adapted, with non-vaccine types becoming more prominent. The researchers emphasize that continued monitoring is essential. Without ongoing surveillance to track these shifts in serotype distribution and antibiotic resistance, public health officials cannot know if current strategies are sufficient or if new vaccines and treatments are needed to stay ahead of the bacteria. The work provides a clear, detailed snapshot of a moving target, highlighting the need for vigilance in the ongoing effort to protect children from pneumococcal disease.