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Increasingly vaccine effectiveness studies are being assessed pre-licensure as part of randomised vaccine effectiveness trials (called phase 3b trials). With measurement of VE under ordinary conditions of PH program
The traditional approach has been to evaluate vaccine effectiveness (VE in the field) post licensure (known as phase 4): to ensure that VE is maintained once a vaccine is introduced into PH practice. This may involve pre-planned evaluations or it may involve outbreak investigations which is often an outbreak in a highly vaccinated population. This may be due to several reasons:
1.Problems in vaccine delivery. A breach in the cold chain due to failure to store a vaccine at the appropriate temp.
2.Epidemiological factors: There may be changes in the pathogen, for example antigenic shift of influenza resulting in a decrease in current VE
4.Low protective efficacy: The vaccine may have been administered to a different target population. For example pneumococcal vaccine phase 3 VE trials were conducted among young, healthy populations such as students. However. the vaccine post-licensure was targeted at the elderly, where VE was then found to be lower. Alternative patterns of use such as the simultaneous use of vaccines or other medicines may affect VE in PH practice eg following the death of a peace corps worker from rabies, who had been given rabies vaccine while taking chloroquine, it was realised that chloroquine inhibits the immune response to id admininstered rabies vaccine.