Read this and answer the question. If the question is unanswerable, say "unanswerable".

A Federal Trade Commission report issued in 1958 attempted to quantify the effect of antibiotic development on American public health. The report found that over the period 1946-1955, there was a 42% drop in the incidence of diseases for which antibiotics were effective and only a 20% drop in those for which antibiotics were not effective. The report concluded that "it appears that the use of antibiotics, early diagnosis, and other factors have limited the epidemic spread and thus the number of these diseases which have occurred". The study further examined mortality rates for eight common diseases for which antibiotics offered effective therapy (syphilis, tuberculosis, dysentery, scarlet fever, whooping cough, meningococcal infections, and pneumonia), and found a 56% decline over the same period. Notable among these was a 75% decline in deaths due to tuberculosis.

What period did the report issued in 1964 cover?
unanswerable