The measles outbreak in the United States is now the largest since the disease was declared eliminated here 19 years ago. The return of this scourge has been driven by one factor in particular: misinformation, spread by vaccine critics, which scares parents into not immunizing their children.

Along with rumors that vaccines cause autism or that the trace amounts of mercury and aluminum in them are dangerous — falsehoods that were long ago debunked — have come innuendos aimed at deeply religious parents.

Vaccines, the activists say, contain ingredients made from pigs, dogs, monkeys and aborted fetuses. Indeed, most of those assertions are based in fact. Ingredient lists published by the Centers for Disease Control and Prevention and the Institute for Vaccine Safety at Johns Hopkins show that vaccines may contain these elements (although any residual DNA is present only at the parts-per-million level).

Nonetheless, vaccination is endorsed by top Jewish and Islamic scholars, and by the Vatican. Religious authorities have meticulously studied how vaccines are made and what is in them, and still have ruled that they do not violate Jewish, Islamic or Catholic law.

Although no vaccine is without side effects, immunization is one of the greatest advances in medicine. The World Health Organization estimates that vaccines have saved more than 10 million lives in just the last decade.

“Since it is proven that vaccines are effective to prevent the spread of disease, it is an obligation upon every father to vaccinate his children,” Rabbi Moshe Sternbuch, vice president of the Rabbinical Court in Jerusalem, recently wrote in an open letter to the dean of a major Orthodox yeshiva in the United States.

Vaccines are highly purified, but they still may contain isolated cells or traces of DNA from the human or animal cells they were grown in. Those “growth media” include cell lines originally derived — often decades ago — from monkey or dog kidneys, moth caterpillars, calf blood, or the immature tissues of aborted human fetuses. (The widely circulated assertion that vaccines contain rat DNA is untrue.)

Some vaccines grown in eggs or using dairy products contain residual egg or casein proteins. And in some vaccines, the manufacturers add small doses of gelatin made from pig skin to prevent damage from heat or freeze-drying.

As New York’s current measles outbreak has spread, the more obscure ingredients of the measles vaccine — which is often delivered in a shot mixed with vaccines against mumps, rubella and chickenpox — have become an issue for some Orthodox Jews.

“The Vaccine Safety Handbook,” published by the anti-vaccine group Peach, has several pages aimed at them, including lists of vaccine ingredients derived from animals that Jews are forbidden to eat.

But kosher dietary laws are “just a total nonissue” with regard to vaccines, said Dr. Naor Bar-Zeev, a professor of international health and vaccine science at the Johns Hopkins Bloomberg School of Public Health. “All these complex laws apply to food ingested by mouth and are not in any way relevant to injected material.”

Observant Jews may inject insulin derived from pig pancreas or have pig valves implanted in a failing heart, Bar-Zeev noted. They may also take oral vaccines, such as those against rotavirus, polio and cholera, even if they contain pork gelatin, because they are considered medicine, not food.

Some Jewish scholars have also ruled that “denatured” substances like gelatin are not subject to the same restrictions as pork flesh.

Vaccine ingredients are not just an issue among Orthodox Jews. Because Islam also forbids eating pork, alarms about gelatin and porcine viral DNA have hampered vaccination in some Muslim countries.

Some older vaccines contained cow gelatin, but manufacturers changed to pork after studies found it triggered fewer dangerous reactions in children with gelatin allergies. In a typical vaccine dose, only about 0.03 of a teaspoon is gelatin.

In 1995, a meeting of 112 leading Islamic scholars considered many ingested substances, including alcohol, rennet and even nutmeg, and approved the use of porcine gelatin in medicines.

“Gelatin formed as a result of the transformation of the bones, skin and tendons of a judicially impure animal is pure, and it is judicially permissible to eat it,” the Islamic Organization for Medical Sciences ruled.

Ingredients were not the central focus of opinions recently delivered by prominent Orthodox scholars, including Sternbuch and Rabbi Asher Weiss, chief authority on medical law for Shaare Tzedek Medical Center in Jerusalem.

Both rabbis not only endorsed existing vaccines but said mandatory vaccination was justified, as was excluding unvaccinated children from yeshivas.

Weiss reiterated the commandment that children must be removed from danger, and Sternbuch cited the principle of pikuach nefesh, which holds that, to save a life or prevent permanent organ damage, any transgression against Jewish law other than idolatry, incest or murder is permitted.

More than 200 years ago, Weiss noted in a recent article, Rabbi Israel Lifschitz, author of a famous commentary on oral Jewish law, declared that Dr. Edward Jenner, the English inventor of the smallpox vaccine, was “one of the righteous among nations” for saving thousands of lives.

The current measles epidemic among Orthodox Jews in Israel, Britain and this country was triggered in part by a pilgrimage last fall to the Ukrainian grave of Rabbi Nachman, founder of the Breslov branch of Hasidism.

Nachman was himself a strong vaccination advocate. Failing to vaccinate children against smallpox before they were 3 months old “is like spilling blood,” he wrote.

Earlier generations of Orthodox scholars had ruled that vaccination was “permissible and proper,” Weiss also said, even with the crude early vaccines that sometimes killed recipients.

Vaccines against viral diseases are made from viruses, which are just protein shells containing short stretches of DNA or RNA and can multiply only when grown in broths of live cells. Those cells are unusual in that they must be “immortal” — that is, able to replicate for decades without suffering “cell death,” the aging process.

(The best-known example is HeLa cells, which were isolated from a tumor in a woman named Henrietta Lacks, who died in 1951. More than 50 million tons of HeLa cells have since been grown for use in cancer research.)

The cells also must be free of cancer and viruses, which is one reason the ancestor cells come from fetuses that have never been exposed to pathogens — fetuses that were removed in sterile surgical environments, not from miscarriages.

Several common childhood vaccines are grown in the MRC-5 and WI-38 cell lines, which are fetal cells that, had they matured, would have become tissues in lungs — the organ in which diseases such as chickenpox and rubella first proliferate. The MRC-5 line originated with a male fetus aborted in Britain in 1964 because the mother suffered psychiatric problems, and WI-38 came from a female fetus aborted in Sweden in 1962 because the parents felt they had too many children.

In 2005, replying to a request for guidance from Children of God for Life, a Florida-based Catholic group, the Vatican said vaccines grown in those cells continued to pose ethical problems “even though this evil was carried out 40 years ago.”

Catholics, the Vatican said, must choose alternative vaccines if they exist and press vaccine companies to make alternatives.

Nonetheless, because there are no alternatives, the use of existing vaccines — particularly against rubella, the Vatican ruled — was “morally justified” because of the higher need to protect children and pregnant women.

The Vatican reiterated that position as recently as two years ago in guidelines for Catholic health workers.

Mormons, Episcopalians, Lutherans and many other Christian denominations endorse vaccines, require them in their schools and distribute them at their missionary hospitals.

High authorities of all other major religions back vaccination, according to a study by Dorit R. Reiss, a medical law expert at the University of California Hastings Law School, and a paper in the journal Vaccine by John D. Grabenstein, an employee of Merck’s vaccine division.

Among Buddhists, the Dalai Lama has personally given polio vaccine to children to further the world polio-eradication drive. One of the first accounts of variolation — an ancient form of smallpox prevention — was from an 11th-century Buddhist nun, who blew ground smallpox scabs into the noses of her patients.

Although the Vatican would like vaccine companies to replace old cell lines with new ones, that is extremely unlikely, experts said. Human fetal tissue would presumably still be required.

The multiyear testing process also would have to start all over again — and anything other than a product perfect the first time could endanger the thousands of infants it would have to be tested in.

“It would probably cost a vaccine company over $1 billion,” said Dr. Paul A. Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “And they’d be competing against themselves. There is absolutely no incentive.”

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