Just looking at vaccination schedules for other countries- why do so many start vaccines at 3 months rather than 2? Seems like it would be beneficial for babies to have protection asap, but even countries with the lowest infant mortality rate don't usually do any shots except hep B until 3 months. Why?
Jamama, If you go here you can view all of the EU countries' vaccine schedules. Australia's here. Most of them have a 2, 3, 4 month or 2, 4, 6 month schedule for the primary series. I chose Denmark for an example of the 3, 5, 12 month DTaP (and other routine infant vaccines) because there is good information collected there. When Denmark was using the whole-cell pertussis vaccine, they actually had a schedule of 5 weeks, 9 weeks and 10 months.
In 1997, they introduced DTaP at the 3, 5, 12 month schedule because of the introduction of other routine infant immunisations along with a DTaP-IPV combination vaccine. My impression, from the literature, is that it came down to catchment and ease. However, a longitudinal survey of this schedule has demonstrated an increase in hospitalised pertussis cases for younger infants, but not for older ones. The culprit being the more delayed and protracted schedule for the primary series.
one important thing to keep in mind when comparing the vaccination schedules in different countries are things like maternity leave and population density. In Sweden, as an example, parents get 18 months of parental leave, breastfeeding rate is very high and population density is low (Sweden is about the size of California, but only has a quarter of its population). So if most children are breastfed at home with mum, they are at a very low risk for diseases spread where the child density is high and by the time these kids enter daycare, they are fully vaccinated.