This is taken from the Official Journal of the American Academy of Pediatrics.
Published online May 17, 2010

A study just published in Pediatrics looked at the incidence of fever after immunizations among babies who were exclusively breastfed, partially breastfed, and formula fed.

In a sample of 450 babies, they found that fever was reported for 25% of the exclusively breastfed babies, 31% for the partially breastfed babies, and 53% for the formula fed babies. So not breastfeeding roughly doubled the incidence of fever compared to exclusively breastfed babies.

Why would this be? The authors note that different responses to viruses among breastfed and not-breastfed babies. They also note that it could be that babies’ feed more frequently for comfort after an immunization, thereby taking in more calories in the post-immunization period – a pattern which does not occur with formula fed babies. Reduced caloric intake is associated with higher levels of pro-inflammatory factors.