Both the Finnish and the U.S. bird flu vaccines are made by CSL Seqirus but are different vaccines. In the summer, U.S. officials stockpiled 4.8 million doses of the vaccine that officials said was effective against circulating H5N1 strains. In October, the U.S. issued a $72 million grant to three manufacturers to double the nation's stockpile.

Two of the four cases are in Lubbock, which hasn’t seen a case in more than 20 years. Meanwhile, measles vaccination rates in Texas have fallen over the last four years.