GLOBALLY, pneumonia is the single largest infectious killer of children. Each year, pneumonia is estimated to cause about 900 000 child deaths. Yet effective tools and knowledge currently exist to both prevent and treat this condition. So why are children still suffering from and dying of pneumonia?
Factors such as air pollution due to biomass-based cooking fuels, household crowding and malnutrition, and low immunisation coverage all increase a child’s risk of developing pneumonia. Limited access to affordable and timely health care results in children dying of this disease.
Pneumonia mortality in children has been falling globally due to improved nutrition, better housing and sanitation, and better access to health care; however, a substantial burden remains. While many children across the world get pneumonia, the majority of deaths occur in low- and middle-income countries.
An important tool at our disposal is the pneumococcal conjugate vaccine. This is a relatively simple intervention that effectively reduces the occurrence of severe pneumonia in children. Vaccination is an essential way to save lives of children who are unable to access or afford health care, and to prevent this disease in the population most at risk. For this reason, vaccines can reduce disparities in pneumonia outcomes within and between countries.
While there is clear evidence from controlled trials that the pneumococcal conjugate vaccine works well in children, we also need to evaluate the vaccine in a range of real world settings. Such evaluations account for important factors that influence the success of a vaccine program. These include aetiologic fraction of pneumonia due to pneumococcus, the dynamics of pneumococcal transmission (including circulating pneumococcal strains) and the strength of the health system in delivering the intervention.
In Australia and the US, the introduction of the 7-valent pneumococcal conjugate vaccine in children resulted in unexpectedly large reductions in disease among adults and the elderly. Targeting children, who were primarily responsible for spreading pneumococcus, resulted in reduced transmission of vaccine strains in the community and widespread reductions in disease beyond those who were vaccinated; that is, herd immunity. However, a proportion of the initial gains have now been supplanted by the emergence of strains of pneumococcus not covered by the vaccine — a phenomenon known as serotype replacement. This reinforces the need for ongoing surveillance and evaluation of the vaccine’s full impact.
To facilitate this type of evaluation, we are working with researchers and public health professionals in Laos, Mongolia and Papua New Guinea to measure how well the pneumococcal conjugate vaccine is performing as it is introduced to these three countries within the Asia–Pacific region, which have a high pneumonia burden.
Evaluations of pneumococcal conjugate vaccine’s impact in low- and middle-income countries are uncommon because conducting this research can be challenging.
The existing surveillance system for evaluating pneumococcal vaccine impacts, established by the World Health Organization, has so far yielded limited results in the Asia–Pacific region. It uses methods derived from high-income countries, requiring the collection of large numbers of blood samples, which is not routine care for pneumonia in many low-income settings. This highlights the importance of integrating local expertise in adapting both interventions and disease surveillance methods to different settings.
In 2018, the WHO surveillance system reported just 44 cases of pneumococcal disease in the South-East Asian region and 77 cases in the Western Pacific region. Such a small sample is unlikely to properly represent the hundreds of thousands of cases in the region, or be able to describe national or subnational changes in disease patterns following vaccine introduction.
Our research uses pneumococcal carriage studies to evaluate vaccine impact. Pneumococcus is commonly carried by children in the nasopharynx. Nasopharyngeal carriage is a prerequisite for infection and the primary means by which pneumococcus is transmitted in the community. Therefore, reductions in nasopharyngeal carriage of pneumococcus vaccine strains are likely to correspond with reductions in disease due to these strains.
One of the key things we want to measure is the degree to which vaccine introduction can protect children and adults who have not been vaccinated (herd immunity).
The presence and degree of herd immunity has two important policy implications. Firstly, it radically improves the cost-effectiveness of the vaccine, helping policy makers to justify the public funding of the vaccine. Secondly, understanding this phenomenon can help policy makers design vaccine programs in such a way as to take advantage of this effect.
In the UK, the Joint Committee on Vaccine and Immunisation recommended switching from giving children three doses to two doses of the vaccine, a move that represents a significant cost-saving for a relatively expensive vaccine. This schedule (one dose at 3 months and another at 12 months) relies on herd immunity to protect children under the age of 12 months who do not have sufficient individual immunity to protect them from disease.
This change is supported by modelling that predicts minimal impact on disease outcomes. However, patterns of pneumococcal transmission and disease vary considerably across settings, requiring research for each. By conducting pneumococcal carriage studies in three very different countries, which have contrasting vaccine programs and disease patterns, we can determine what factors maximise the herd effect necessary for the use of reduced dose schedules.
Furthermore, by studying the vaccine in real world settings, we can monitor which children are and are not accessing and benefiting from vaccination programs. A vaccine program that fails to reach those children who are most at risk of pneumonia will only exacerbate the existing inequities in the burden of pneumonia.
Indeed, our initial findings indicate high variability in vaccination coverage across communities, especially in Papua New Guinea. Our final results will also tell us how this affects disease transmission. Hence, we can use these data to improve health as well as to highlight issues with equity and social inclusion.
However, a remaining challenge is our understanding of serotype replacement in the Asia–Pacific region. While our research provides important information about the circulating non-vaccine strains, we do not know the degree to which these strains are causing disease. Previous research, predominantly from high-income countries, has suggested that many non-vaccine strains are likely to be less invasive. However, there are striking differences in the degree of serotype replacement disease in different countries and different populations, and the reasons for this are not well understood. Therefore, ongoing evaluations of disease, in addition to carriage, are required.
Dr Jocelyn Chan is a public health registrar and PhD candidate at the Murdoch Children’s Research Institute.
Professor Fiona Russell is a paediatrician with qualifications in public health and epidemiology. She is a Team Leader in the Asia–Pacific Health group at the Murdoch Children’s Research Institute.
The statements or opinions expressed in this article reflect the views of the authors and do not represent the official policy of the AMA, the MJA or InSight+ unless so stated.