Systematic literature review and meta-analyses of the benefits and risks of measles vaccination below 6 months of age

Background
This report summarises the results of an updated systematic review of the literature and meta-analyses into the benefits and risks of giving a first dose of measles containing vaccine (MCV) to infants below 6 months of age in order to inform the discussion about an optimal age of the first dose in different epidemiological settings.
Methods
We performed a systematic literature review for studies in which standard titre MCV was given to infants <6 months of age. We extracted data on the following outcomes: immunogenicity (humoral and cellular), vaccine efficacy or effectiveness (VE), duration of immunity, blunting and safety. Where appropriate, meta-analyses and meta-regression was performed. We compared results with results from our 2015 review of MCV1 <9 months of age. Quality of all included studies was assessed using the GRADE methodology.
Results of the search and selection
From an initial search carried out in June 2015, 867 references were identified. Following an updated search in April 2017, an additional 186 references were identified. A total of 19 studies from both searches were included in the review.
Results of the review of included studies
The proportion of infants seroconverting increased from 50% (95% CI 29-71) at 4 months of age to 67% (51-81%) at 5 months of age. The proportion of infants seroconverting was found to be further dependent on the vaccine strain used. Seropositivity also increased with age and was strain dependent. GMTs were higher after MCV<6 compared to ≥6 months of age, but results for MCV<6 months were derived from only two studies. There was limited evidence on cellular immunity, vaccine efficacy and effectiveness, duration of immunity, blunting and safety to draw conclusions.
Conclusions
We found humoral immunity after MCV1<6 months of age was dependent on age of MCV1 and presence of maternal antibodies. There was limited evidence available for cellular immunity, vaccine effectiveness, duration of immunity, safety and blunting following MCV1<6 months of age.