Rules for chemical formulas (Back to search) 1  Enter a sequence of element symbols followed by numbers to specify the amounts of desired elements (e.g., C6H6). 2  Use correct case for element symbols. 3  Elements may be in any order.  If only one of a given atom is desired, you may omit the number after the element symbol.
Half of all varicella zoster cases occur in children between the ages of 5 and 9. It has been estimated that, before the chickenpox vaccine was introduced in the U.S. in 1995, only 10 percent of Americans over the age of 15 had never had chickenpox.
But the more telling illustration is the accompanying graph, which shows the sharp and virtually permanent drop in polio incidence in the U.S. after mass vaccination began. The Salk vaccine was declared safe and effective in 1955. An oral vaccine developed by Albert Sabin was first tested in 1957 and went into widespread use in 1963.