SÃO PAULO — Hoping to stave off another deadly outbreak of yellow fever, Brazil’s government announced on Tuesday that it planned to vaccinate the entire country against the mosquito-borne virus by April 2019.

The country of 208 million is grappling with the worst outbreak of yellow fever in decades. So far this year, 300 people, including several tourists, have died from the virus, which has hit the outlying areas of the country’s largest cities, Rio de Janeiro and São Paulo, particularly hard, threatening to become this country’s first-blown urban epidemic since 1942.

“We are going to act preventively instead of reacting with emergency measures as we have done,” Health Minister Ricardo Barros said at a news conference in Brasília, where he announced that health officials would need to vaccinate an additional 77.5 million people to reach the country’s entire population by April of next year.

The disease, normally found in the Amazon River Basin, broke out of its usual pattern in early 2016 and started moving south and east toward the country’s largest cities, prompting the declaration of a public health emergency last year.