Here is the timeline for any who are interested:
In September of 1958 vaccines and serums containing plasma proteins (i.e. gamma globulin and albumin) were exempted from the prohibition:
"While God did not intend for man to contaminate his blood stream by vaccines, serums or blood fractions, doing so does not seem to be included in God's expressed will forbidding blood as food. It would therefore be a matter of individual judgment whether one accepted such types of medication or not." (The Watchtower September 15, 1958 p. 575)
In September of 1961 the prohibition was restated in draconian terms that would have precluded all treatments involving blood:
"In view of the emphasis put on the use of blood in the medical world, new treatments involving its use are constantly being recommended. But regardless of whether it is whole blood or a blood fraction, whether it is blood taken from one's own body or that taken from someone else, whether it is administered as a transfusion or as an injection, the divine law applies. (The Watchtower September 15, 1961 pp. 558, 559 emphasis mine)
Two months later, they were questioned about this:
Since the Bible forbids the eating of blood, how are Christians to view the use of serums and vaccines? Has the Society changed its viewpoint on this?—J. D., U.S.A. (The Watchtower November 1, 1961 p. 670)
The answer read in part:
"As to the use of vaccines and other substances that may in some way involve the use of blood in their preparation, it should not be concluded that the Watch Tower Society endorses these and says that the practice is right and proper. However, vaccination is a virtually unavoidable practice in many segments of modern society, and the Christian may find some comfort under the circumstances in the fact that this use is not in actuality a feeding or nourishing process, which was specifically forbidden when God said that man was not to eat blood, but it is a contamination of the human system." (Ibid - emphasis mine)
So you can see that they were not willing to stand by the statement made earlier in September of the same year. Apparently one of the writers was simply an ignoramus. (Or at least more of one than his peers.)