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Kathleen Wiederman is not staunchly against vaccines. She simply believes it is better for her child to naturally battle an illness than to be vaccinated against it.

"Doctors don't know everything," said the 42-year-old recruiter, who prefers alternative medicine and gave birth at her home in the well-heeled Virginia suburbs without the aid of a pain-killing epidural.

At first, she and her husband agreed on the matter, but when their marriage ended, he pushed for their daughter to get some of her recommended vaccines and Wiederman relented.

Now her daughter is five and has had a handful of shots, including against chicken pox and measles, but not polio.

And if her child gets sick?

"Then we treat it however you need to treat it and work through it," she told AFP.

Wiederman, who has a law degree, is among a growing number of Americans who oppose vaccines, raising concerns about a resurgence in contagious diseases like measles and whooping cough.

Vaccine hesitancy is increasingly common, and not only when it comes to infant and childhood immunizations, experts say.

Two in three working age adults refuse to get the annual flu vaccine and the same proportion of parents decline the human papillomavirus (HPV) vaccine for young adolescents, according to the US Centers for Disease Control and Prevention.

"The people we are concerned about are the people who are hesitant. The general demographic is well-educated and upper middle class," said Barry Bloom, a professor of medicine at Harvard University.

"I think they are on the rise everywhere."

In recent years, reports linking vaccines to autism have been debunked, but fears of adverse events -- which experts say are rare -- have proven difficult to erase.

Some parents are troubled by the increasing number of vaccines children are given, which have risen from seven in 1985 to 14 today, a result of medical advances, according to the Centers for Disease Control and Prevention.

"I was stunned by the number of vaccines," said Alina Scott, a 37-year-old project manager and mother of a two-year-old son.

Scott said she began reading everything she could find on the topic, even before her child was born, and decided that vaccines were not for them.

"This lasted until about a year ago, when I just felt like I wasn't finding any new information. It's like I hit the end of Internet," she said. "I don't think we will be vaccinating any time soon."

- Religious exemptions -

Nearly all US states require a standard list of immunizations before children can enter school, but they allow religious exemptions for vaccines. Some allow parents to opt out for personal reasons.

Some measles outbreaks in recent years, including in the Orthodox Jewish community in New York City, have been linked to parents refusing vaccines.

"Today you are allowed to have philosophical reasons not to vaccinate and I think that is crazy," said Anne Gershon, director of the Division of Pediatric Infectious Disease at Columbia University Medical Center.

"The reason is that it hurts many people. It is not just your child."

Some young people cannot get vaccines, including those with cancer or immune diseases, and very young infants are vulnerable to pertussis, or whooping cough, until the age of two months when they can get begin to get doses of the vaccine.

Particularly with measles, one of the most contagious diseases, outbreaks will occur unless 94 percent or more of the population is vaccinated, according to Bloom.

Nationwide, vaccination rates among US kindergarteners have stayed high -- near 95 percent.

But a 2011 poll in the journal Pediatrics found that one in 10 parents did not stick to the recommended schedule of vaccines for their child, and a quarter of parents had doubts about vaccine safety.

The United States typically sees about 60 cases of measles per year.

"We don't have a crisis, but nonetheless the trend is going up and the number of immunizations is going down," said Bloom.

- Flu shots, cancer vaccines -

Another trend is resistance to vaccines like the annual flu shot, experts say.

The CDC said in February that two-thirds of adults aged 18 to 65 had not had their seasonal shot, and that hospitalizations in this age group had doubled over last year. Deaths from flu complications were also far higher than usual.

Doctors are also surprised at how many decline the three-dose shot to prevent HPV, a sexually transmitted infection which can lead to cervical cancer in women and cancers of the head, neck, penis and anus in men.

The vaccine is recommended for school age boys and girls before they become sexually active, and can be given as early as age nine.

Just one in three women aged 19 to 26 had been vaccinated in 2012, and just 2.3 percent of men, the CDC has reported.

"I think for physicians, the idea that vaccines could prevent cancers seems phenomenal," said Anne Schuchat, Assistant Surgeon General and director of the National Center for Immunization and Respiratory Diseases.

"It has been a surprise it has not been going like hotcakes."

When it comes to how to communicate the benefits of vaccines to a skeptical public, experts are stumped.

In Bloom's view, vaccines have fallen victim to their own success.

"If they have never seen a kid blinded from measles, or mentally retarded from pertussis, it is very hard in this wonderful, happy, affluent world of kindergartens and first and second grades to see that there is a problem that vaccines are preventing."