USBN #173 February 1945 p3
" Regarding your question about vaccination: these are technical matters which have not been specifically mentioned in the teachings , and consequently the Guardian can not make any statement about them. No doubt medical science will progress tremendously as time goes by, and the treatment of disease becomes more perfect.
"It is advisable to use both the Baha'i dates, according to the Baha'i Calendar, and the usual Gregorian dates as well. The friends at present are free to do as they please.
"The Guardian hopes that along with whatever other studies you take up, you will continually study the teachings and endeavor to acquire a profound knowledge of them. The importance of young Baha'is becoming thoroughly steeped in every branch of the teachings can not be over-emphasized, as they have great teaching tasks ahead of them to accomplish."
(From letter to John C. Eichenauer, Jr., December 24, 1943)