The most serious flu pandemic should prompt immediate isolation measures, including sending students home from school for up to three months and quarantining households with sick family members, according to federal guidelines issued Thursday.

Because it would take four to six months to prepare a vaccine to protect against a pandemic flu, the guidelines are critical to restricting the virus.

The best alternative to a vaccine “is to try to slow down the spread and buy some time,” said Dr. Julie Gerberding, the director of the Centers for Disease Control and Prevention.

The guidelines were created, in part, because of continuing concerns over bird flu, which has spread through Asia, Europe, the Middle East and parts of Africa. The virus, which rarely infects humans, has not been detected in North America.

The guidelines from the Department of Health and Human Services grade the severity of flu epidemics like hurricanes.

The most serious outbreak, classified as a Category 5, would be on par with the 1918 Spanish flu pandemic, which killed tens of millions of people. Such pandemics would call for postponing or canceling public gatherings, such as sport and theatrical events; closing child care centers and staggering work schedules to thin crowded offices.

A regular seasonal flu, which has a fatality rate of less than 0.1 percent, would be classified as a Category 1. The guidelines’ only recommendation is that sick people voluntarily stay home until they recover, possibly up to 10 days.

Dr. Jonathan Fielding, Los Angeles County’s public health director, said guidelines appear to be appropriately measured.

“We always want to err on the side of caution,” he said. “This document, on first reading, appears to do that.”

Local health officials would have the ultimate authority to implement the measures, but the CDC recommended that they start quickly with the first cluster of confirmed cases in a state or region.

A poll by the Harvard School of Public Health conducted in conjunction with the report found that 57 percent of the people surveyed said they would have serious financial problems if they had to miss work for more than a month. About a third said they would ignore the restrictions if their employers asked them to come to work.

If schools and day care closed for a month, 93 percent of adults said they could arrange for care for their child, but about 60 percent said that at least one employed person would have to stay home to take care of that child.

Economic concerns led to the omission of travel restrictions, said Dr. Marty Cetron, who helped develop the guidelines at the CDC. “The trucks that cart the chlorine we need to purify water may have to move across state lines,” he said. “We want to preserve the functioning of civil society and work to the extent possible. … If we are in the setting of a Category 5, then we can re-evaluate.”