Letter to the editors

Sirs, In the January issue of your journal, you published two articles as well as an editorial related to observational studies investigating impact of infant vaccination on mortality in low-income countries (Aaby et al. 2007; Jensen et al. 2007). We feel it is important to correct some misrepresentations of a study conducted in Papua New Guinea (PNG) and published in the International Journal of Epidemiology (IJE) (Lehmann et al. 2005), particularly as this is not the first time this has happened. Aaby et al. (2007) on page 20 of their publication in Tropical Medicine and International Health (TMIH) state that These studies did not exclude children for lack of information, nor were they likely to obtain information on vaccination status post-mortem . In our manuscript (Lehmann et al. 2005) we wrote on page 146: To avoid bias towards surviving children only those born under demographic surveillance who were registered on the database within 60 days and who survived the neonatal period were included. We aimed to maximise the chances of complete registration of deaths as well as immunisation by censoring children at time of first migration out of the area under demographic surveillance . Furthermore, on page 140, we wrote that all clinic cards held at each health centre/hospital for all outreach clinics were brought into the office monthly for entry of details of immunisations and clinic visit dates. This included cards belonging to children who had died, since nursing staff kept their cards as well as those belonging to migrants or non-attenders for one year after their last attendance. If no identity or social number was recorded on a clinic card, office staff gave demographic details to reporters to provide the identity number. This was done repeatedly until identity and place of residence was established. Following this intensive process of identification and errorchecking, if there was no clinic card linked to a child on the demographic database, it was concluded that the child had not been vaccinated . Moreover, on page 20 Aaby et al. (2007) write: In summary, in studies using retrospective updating, the MR for vaccinated compared with unvaccinated children was only slightly below 1 in studies with a short interval between visits.In contrast, the MR was very low in studies with long intervals and potential survival bias . In our IJE article, we had clearly indicated (as quoted above) that we undertook monthly collection of immunisation data and independent monthly demographic surveillance. During fortnightly meetings attended by all village reporters, the reporters communicated with each other and office staff about important demographic events (including deaths or migrations), which were related to independently collected immunisation data from health centres. Because of the monthly demographic surveillance there were no long intervals between home visits. Importantly, we state in the IJE article on page 147 that To allow more stringently for health service utilisation, we also included an analysis restricted to BCG recipients, which confirmed protection afforded by DTP and a dose-response to DTP . This analysis only included children for whom we had a vaccination card held at a health centre and which we collected monthly. The mortality ratios for this analysis have not been commented on by Aaby et al. There are several errors in Table 2 of the Aaby et al. paper. The last five columns labelled Burkina Faso, Malawi, PNG, Bangladesh and India used the Retrospective updating approach and not the landmark approach . In the column related to the PNG study: