The current U.S. guideline for immunizing children against polio, whooping cough, measles and other infectious diseases is safe, but should still be monitored, federal health advisers said on Wednesday.

In what they called the most comprehensive review to date, scientists at the Institute of Medicine (IOM) said there is no evidence that giving children vaccines according to the recommended timetable causes other problems such as autism or asthma.

IOM, part of the National Academies, a federally-charted group of scientific advisers to the government, said it hopes the findings would reassure parents, doctors and others, but it recommends that the research continue.

Requested by U.S. health officials, the year-long review of existing research underscores the lingering concerns some people have about the vaccines, especially the many shots babies and toddlers receive.

The findings come as the nation wrestles with various outbreaks, including an influenza epidemic. Several U.S. states are also grappling with record spikes of whooping cough.

Federal vaccine guidelines recommend 24 immunizations by age 2, and sometimes children get up to five shots in one doctor's visit.

While most people follow the recommended timetable, IOM said about 1 percent of Americans refuse all vaccines.

The reasons vary. Some object for religious reasons but others are concerned that underlying medical conditions could raise the risk of possible complications from the injections. Others worry potential harms outweigh the benefits or simply mistrust the government, which reviews and approves vaccines before they can be marketed.

Still, most parents comply.

Pamela Maslen, a registered nurse and lactation consultant in Silver Spring, Maryland, said her work overseas influenced her decision to follow the recommendations when her first daughter was born nearly five years ago.

"I pretty much decided I wanted to keep on the schedule because I knew we would be moving, and I didn't want her to be susceptible to anything," said Maslen, 35, who has two daughters and is expecting her third child soon.

Some parents still worry

IOM's panel of independent scientists looked at the schedule of immunizations and all available scientific data to determine safety. They also reviewed CDC and the Food and Drug Administration databases that track side effects.

Yet suspicions over vaccines have continued for years. Nearly 40 percent of U.S. parents have some mistrust of childhood vaccines, the CDC has said.

Some suspicions arose over autism and thimerosal, a mercury-based preservative once used in many U.S. vaccines that was later removed from childhood vaccines. No studies have shown a clear link, and IOM said in 2004 that researchers should look elsewhere for a cause to the disorder.

"The concerns are certainly still out there," said Cassandra Jessee, 39, who opted to "delay" the vaccines for her 16-month-old son by spreading them over several months rather than one doctor's visit.

"It means more co-pays and doctors appointments, but to me it is worth it," she said.