It's not clear that smallpox-infected blankets even worked. It’s also not clear whether or not the attempt at biological warfare had the intended effect. According to Fenn’s article, the Native Americans around Fort Pitt were “struck hard” by smallpox in the spring and summer of 1763...
The massacre was a direct result of thedeathsof a number of Utes, including Chief Chico Velasquez, after they became infected with smallpox. The Native Americans, who had never been exposed to the disease on the North American continent, caught smallpox from blankets given to them by settler...