Here are some of the latest health and medical news developments, compiled by editors of HealthDay:

Doctors Upset About Losing Money on Vaccinations: Survey

About 40 percent of U.S. doctors who vaccinate privately insured children are thinking about withdrawing the service, because they lose money on it, according to a survey of about 800 pediatricians and family physicians.

About half the respondents said they'd delayed buying at least one vaccine due to the cost, and almost 20 percent said they felt strongly that they weren't adequately reimbursed for the purchase and administration of vaccines, the Associated Press reported.

A second survey of 76 doctors in five states found major differences between what doctors pay for vaccines and what their reimbursement is from private insurers. For example, 10 percent of doctors lost money on a recommended infant vaccine, while others made almost $40 per dose for giving the same shot.

Both studies were published in the December issue of the journal Pediatrics.

There's no evidence that large numbers of doctors are withdrawing vaccination services because of financial concerns, according to experts. But health officials are worried about that possibility, because it would mean fewer people would receive recommended shots, the AP reported.

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294,000 Children Sickened by Tainted Dairy Products: China

A total of 294,000 children in China fell ill after consuming melamine-tainted dairy products, and 154 of them remain in serious condition, the country's health ministry said Monday in a statement posted on its Web site.

That new figure is almost six times higher than the government's previous figure of 53,000 that was given in September, Agence France-Presse reported.

Officials also said there may be more than the previously-announced four deaths, because six more deaths since Sept. 10 may be linked to consumption of melamine-tainted milk.

Of the 294,000 children who suffered urinary tract problems, 51,900 were admitted to hospital, the health ministry said. Of those, 861 remain in hospital and 154 were "serious" cases, AFP reported.

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Scientists Halt, Reverse Heart Disease in Mice

Researchers who halted and even reversed some of the effects of heart disease in mice say their findings show that microRNA plays an important role in the development of heart disease. MicroRNAs regulate activity of genes.

An international team of scientists found that cells in failing hearts had higher levels of a specific type called microRNA-21. They linked it to a signaling pathway that leads to heart disease-associated tissue damage, BBC News reported.

When the researchers used a chemical to block microRNA-21, they noticed an interruption in this pathway and improved cardiac function in the mice. The findings were published in the journal Nature.

The results are "exciting," said Professor Eric Olsen, a University of Texas Southwestern Medical Center researcher who specializes in microRNAs and heart disease.

"This research suggests you can reverse or prevent aspects of heart disease," he told BBC News. "There are already studies in large animals using microRNA inhibitors in heart disease -- I can envisage that in a few years we will see this in human trials."

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Night Terrors Have Hereditary Component: Study

Genetics play a major role in night terrors, according to a Canadian study that included 390 pairs of identical and fraternal twins assessed for the condition at 18 months and 30 months of age.

Among identical twins, there was a 68 percent chance that both twins would have the condition. Among fraternal twins, that likelihood was 24 percent. The researchers concluded that more than 40 percent of night terrors can likely be attributed to genetics, the Canadian Press reported.

The findings were published Monday in the journal Pediatrics.

"The study brings strong evidence that genetics plays a major role," said principal researcher Dr. Jacques Montplaisir, a professor of neuroscience and psychiatry at the University of Montreal, the CP reported.

Children with night terrors experience sudden episodes of extreme fear and screaming an hour or two after they go to sleep. Most children outgrow the condition, said Montplaisir, who noted that environmental factors also play a role in children's risk of night terrors.

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Study Questions Use of Antioxidants to Fight Aging

Antioxidant diets and creams don't slow aging, say British researchers.

It's long been suggested that reactive forms of oxygen called free radicals cause molecular damage responsible for aging. Antioxidants supposedly clean up these free radicals and minimize their damage, called oxidative stress, BBC News reported.

In a new study, University College London researchers genetically manipulated nematode worms (which share many genes with humans) so that their bodies could "mop up" surplus free radicals. But these worms lived no longer than normal nematodes, which suggests that the effect of oxidative stress on aging has been exaggerated.

"The fact is that we don't understand much about the fundamental mechanisms of aging -- the free radical theory has filled a knowledge vacuum for over 50 years now, but it doesn't stand up to the evidence," said team leader Dr. David Gems, BBC News reported. "It is clear that superoxide (free radicals) is involved, but it plays only a small part in the story -- oxidative damage is clearly not a universal, major driver of the aging process."

The study was published in the journal Genes & Development.