BETHESDA, Md. - Baseless Internet-generated scares about a preservative no longer used in childhood vaccines in the United States suddenly are looming as a much more serious threat to public health than diseases like SARS or monkey pox.
   The scare tactics have raised concern among parents about the safety of common childhood vaccines that have caged such once lethal killers as mumps, measles, diphtheria and tetanus.
   And the panic they've triggered among parents has attracted the attention of some of the nation's personal injury lawyers - who see the potential for mega-buck payoffs from claims that vaccines may cause neurological disorders including autism.
   Although debunked by numerous highly credible scientific studies and decried by leading medical groups, those claims have spread over the Internet like wildfire. They have been given added visibility by Rep. Dan Burton, R-Ind., an 11-term conservative who repeatedly has championed the charges at hearings of his House Reform Subcommittee on Wellness and Human Rights.
   How dire is the situation? Consider that in the early 1980s some 15 companies manufactured vaccines for children. Today, only four manufacturers are left in the United States. The others abandoned the market due to escalating liability stemming from baseless claims, which compounded the problem that the economics of the vaccine business have never been good.
   The biggest victims, though, could be children. If history repeats itself, companies could again be driven out of the immunization business. Without widespread availability of vaccines, childhood diseases would stage a mass comeback - afflicting millions of children and sending thousands to premature deaths.
   Before the measles vaccine was developed, the disease killed half a million children in the United States. Compare this with SARS which, to date, has claimed 745 lives worldwide - none in the United States.
   The recent vaccine lawsuits began two years after the U.S. Public Health Service and the American Academy of Pediatrics recommended that thimerosal be removed from infant vaccines.
   The decision was driven by concern about a theoretical risk for some children who may be exposed to levels of mercury that came close to or exceeded the Environmental Protection Agency limit for known safe levels of exposure to methylmercury - a different and more toxic form of mercury than the ethylmercury-based thimerosal.
   This recommendation was made as a precaution. There was no evidence showing thimerosal was unsafe in amounts used in vaccines. However, rather than wait for more data to emerge, the public health community chose to invoke the precautionary principle - otherwise known as ''better safe than sorry'' - and requested that manufacturers eliminate thimerosal in vaccines.
   Methylmercury and ethylmercury are not the same. Although the words ''methyl'' and ''ethyl'' differ by only a single letter, their different effects are illustrated by the difference between ''ethyl alcohol'' and ''methyl alcohol.'' Ethyl alcohol is the sometimes intoxicating ingredient in beer and wine; and methyl alcohol is the highly toxic ingredient in wood alcohol, that can cause permanent blindness after drinking only a thimbleful.
   A recent study on thimerosal exposure was published in November 2002 in the British medical journal The Lancet. Researchers at the University of Rochester tested infants who had received thimerosal-containing vaccines and compared them to infants seen at a clinic that used mercury-free vaccines. Of the 33 infants exposed to thimerosal, all tested below EPA's limit for the more toxic methylmercury exposure, which itself includes a 10-fold margin of safety.
   The phasing out of thimerosal as a preservative in vaccines began in July 1999. If thimerosal were the autism culprit, we should see a dramatic drop in autism rates over the next few years. However, the litigation is trying to rush to judgment before the evidence is in.
   Despite the complete lack of science, the thimerosal lawsuit flood has begun.
   As a result vaccine manufacturers, children's hospitals and physicians alike now face the threat of lawsuits that may jeopardize mass inoculations and punch large holes in the public health armor that shields American children from many of the dread diseases of the recent past.
   Congress can - and should - remedy this emergency situation by passing legislation ensuring that claims against vaccine makers do not circumvent the Vaccine Injury Compensation Program that was enacted by Congress in the late 1980s and has worked effectively ever since.
   This is a reasonable and rational way to ensure America's schoolchildren continue to have adequate protection against the dread diseases that regularly purged their ranks less than five decades ago.