Most of the static fridges in PNG functioned well in maintaining the vaccines at correct temperatures most of the time. But there is clear evidence of vaccines having been exposed to freezing temperatures during transport at multiple locations in the cold chain. The ice packs were presumably too cold when placed in the vaccine carriers, and there was insufficient distance between them and the vials, causing the vaccines to cool below freezing. Short exposure to high temperatures was not a problem despite very high ambient temperatures. The National Department of Health has already begun corrective training for relevant staff, and at least in the short term, it is anticipated that there will be much lower numbers of vaccine freezing incidents.
For over a decade, the use of data loggers has been shown in many countries to be useful. This technology has proved itself again in the challenging environment of PNG, and will hopefully be used repeatedly in the future to monitor for freezing events.
There is surely a wider lesson to be learned from the PNG study–that the entire cold chain system (however well it is installed) is vulnerable to human error. Can this vulnerability be reduced in hot climates by simple or not-so-simple technological solutions? We offer some potential solutions such as changing the presentation of liquid vaccines to a more heat-stable form. The Global Alliance for Vaccines and Immunization is already supporting the introduction of combination vaccines (that include DTP, HepB and Hib vaccines) in many countries. We believe that hot climate countries will increasingly need to avoid cold damage in anticipation of an era when increasing numbers of costly, freeze-sensitive vaccines will be introduced. Investing in studies such as this as a means to identify and solve problems, and to strengthen staff capacity is time and money well spent.