ATLANTA — The US government may increase the number of healthcare and emergency workers to be vaccinated against smallpox, expanding on a more conservative plan put forth last month by experts at the Centers for Disease Control and Prevention (CDC) while stopping far short of vaccinating the entire population, according to officials.

On June 7, The New York Timesreported that the federal government was planning to vaccinate up to 500,000 healthcare and emergency workers. However, a US Health and Human Services (HHS) Department spokesman, Bill Pierce, told The Scientist that the exact number to be vaccinated has not yet been determined.

Donald A. Henderson, chair of the Secretary's Council on Public Health Preparedness at the Department of Health and Human Services (HHS), told the Times that the tentative new plan called for many more to be vaccinated. "We could easily be at a half-million without too much difficulty," Henderson...