As summed up by the NYT: "volunteers were vaccinated against the virus that causes hepatitis B; several months later, the happy volunteers showed a stronger response to the vaccine than the unhappy volunteers."

In scientific terms, the authors found "an association between dispositional positive affect and magnitude of secondary antibody response to hepatitis B vaccination."

The authors speculate about many mechanisms, including neuroendocrine, in which psychology and immunity can be related. They also address various confounding factors like social relationships and exercise. 

There are some important implications. Positive affect can help explain the "interindividual variability in the maintenance of protective immunity over time." In other words, psychology may have to do with why some people lose hep B protection over time and require booster shots.

I wonder, how did the researchers find so many graduate students at their institution who had not been vaccinated before. Surely this points to gaps in the system for universal vaccination.

For the original paper, see http://www.psy.cmu.edu/~scohen/Marsland_etal_BBI_May06.pdf