Officials from Children's Hospital & Research Center Oakland said the hospital has already starting banning visitors under 18.

Stanford Hospital officials said they restricted visitations at their hospitals because children are more likely to get swine flu and they can be infectious for longer periods of time, even after symptoms are gone. The policy will remain in effect throughout the flu season, which usually lasts into spring.

"Our flu season is early, but it has not been catastrophic. This is really a proactive measure, not only for our patients but for our staff, to decrease amount of exposure," said Nancy Lee, Stanford Hospital's vice president for patient care services and chief nursing officer.

Children's Hospital Oakland spokeswoman Diana Yee said the hospital, which just started the restrictions, selected 18 for the cutoff because 18-year-olds generally can provide identification as proof.

"We apologize for any inconvenience to our patients and visitors, but these changes are essential for protecting the entire community during this year's flu season," she said in an e-mail.

The most vulnerable

Children, pregnant women and young adults are particularly vulnerable to the swine flu, a form of influenza Type A, subtype H1N1. The virus has infected millions of people worldwide since April, when it was first reported, and does not appear to be letting up.

A number of other hospitals across the country, including Johns Hopkins Hospital in Baltimore, University of Maryland Medical Center, Concord Hospital in New Hampshire and Emory University's hospitals in Atlanta, have recently banned adolescent and child visitors. The age restrictions range from under 12 to under 18.

Bans elsewhere

Some California hospitals, particularly those in the Central Valley, have limited younger visitors.

Adventist Health Central Valley Network, which operates hospitals in Hanford (Kings County) and Selma (Fresno County), has restricted visitors under 15 years old. Children's Hospital Central California in Fresno, which has had a longtime policy of limiting visitors under 12 at the start of the annual flu season in November or December, enacted its restrictions this year on Sept. 11.

"We thought it would be better to be safe than sorry," said Beverly Hayden-Pugh, vice president and chief nursing officer of the Fresno children's hospital. She said all visitors are screened for flu symptoms before they enter the hospital.

Hayden-Pugh and Stanford's Lee both said they realize the visitation restrictions could be difficult for families and noted exceptions could be made, particularly in cases of a patient's imminent death.

Kaiser Permanente officials said they have encouraged visitors who are feeling ill not to visit patients in their hospitals, but have not imposed any visitation restrictions.

Other Bay Area hospital officials, including those from California Pacific Medical Center in San Francisco, San Francisco General Hospital and Alta Bates Summit Medical Center in Oakland and Berkeley, said they were using inpatient admissions for H1N1 as a guide and simply had not seen enough of the illness to warrant such measures. However, they said, they are routinely evaluating the situation.

"We just see it as particularly hard on our patients and are not certain it would be effective," said Rachael Kagan, spokeswoman for San Francisco General Hospital. "Certainly we could revisit this."

Discussions at UCSF

At UC San Francisco Medical Center, where children under 12 have been banned from visiting patients in high-risk units since the mid-1990s, officials are discussing whether to limit visitors under the age of 16 altogether during this flu season.

Dr. Lawrence Drew, head of UCSF's diagnostic virology laboratory, said he understands hospitals' rationale in limiting young visitors.

"We're at the worst possible time because the kids have all gone back to school, which is the ideal environment from the virus' standpoint," he said.

E-mail Victoria Colliver at vcolliver@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle