6th Global Meeting on Implementing New and Under-utilized Vaccines, 15-17 May 2012
Workgroup 6 - Assessing the disease impact of new and underutilized vaccines
Background
While there has been a rapid increase in the uptake of new vaccines in many developing countries, demonstration of impact is considered essential to secure investments for sustaining the use of these vaccines, both from countries and partner agencies. The newly established network for invasive bacterial vaccine preventable diseases (IB-VPD) and rotavirus gastroenteritis are beginning to generate empiric data that may be used to monitor impact of Hib, PCV and rotavirus vaccines. For Hib and pneumococcal invasive disease cases identified through IB-VPD surveillance, the country specific data need to be individually considered for appropriate analyses. Not all data are appropriate for all analyses thus it is essential that data used to monitor impact meet quality criteria and are used and interpreted appropriately.
WHO in collaboration with partners has recently launched an impact assessment manual for Hib and pneumococcal diseases. The manual describes what types of analyses can and cannot be done with various types of IB-VPD surveillance data. In addition to IB-VPD surveillance data, other types of data can also be used to assess Hib vaccine and PCV impact. These may include surveillance for pneumonia impact, particularly with the use of standardized reading of chest radiographs, and surveillance for changes in the individual and community level circulation of pneumococci colonizing the nasopharynx.
Main topics of discussion
Gold standard studies with several years of pre- and post-introduction high quality surveillance data are ideal, but seldom available. However, the Hib and PCV impact assessment manual provides approaches to assessing impact using data available in countries. While this manual will certainly help, additional, in-person assistance may be required to assist countries in selecting the most appropriate approach and in designing the studies that will provide them with the data they require.
The case-control approach has been widely used to assess impact, but the presentations and discussions during the workshop highlighted some of the limitations of this methodology and the impact of confounders that could influence the results - there was a tendency for many of the confounders to bias the results in favour of higher observed effectiveness
The data most likely to be available to assess impact would be data from surveillance for invasive disease. However, these data are limited in many countries and may not be of sufficient quality or number (cases identified) to assess impact. Also, sufficient data may not be available pre-introduction and the rapid decline in disease rates post-introduction may preclude proper assessment if assessment is delayed.
Since most of the surveillance sites do not have a defined denominator, local data may need to be bridged to data from more sophisticated special studies. Several alternatives are possible but have limitations: these include the use of pneumonia outcomes and cross-sectional studies of nasopharyngeal carriage. The main problem with use of pneumonia outcomes relate to issues around pneumonia definitions and the non-specificity of the available definitions, making data interpretation difficult.
Special studies using NP carriage need to be conducted before and annually for several years post introduction, but need to be conducted using the same methods and during the same season each year; the laboratory methods should allow detection of multiple serotypes. Methods for collection, storage and testing of nasopharyngeal swabs are being updated.
Whatever the method used results of analysis should not be interpreted in isolation, but in the context of other available epidemiological information.
Recommendations
- Establish guidance on conducting, analysing and interpreting impact assessment using nasopharyngeal carriage and pneumonia end points; may need to re-establish the arbitration panel for interpretation of chest radiographs
- Field test methods in a few select sites
- Document experiences gained from using the impact assessment manual to inform further improvements to the document
- Promote greater “democratization” of data so that information can be shared between countries and allow for synthesis of data at regional and global level to better inform countries and stakeholders on impact of these vaccines.