The U.S. government is starting an unprecedented system to track possible side-effects as mass swine flu vaccinations begin next month. The idea is to detect any rare but real problems quickly, and explain the inevitable coincidences that are sure to cause some false alarms.

"Every day, bad things happen to people. When you vaccinate a lot of people in a short period of time, some of those things are going to happen to some people by chance alone," said Dr. Daniel Salmon, a vaccine safety specialist at the Department of Health and Human Services.

'Part of what we hope is that it will teach us something about how to monitor the safety of all medical products quickly.'— Dr. Richard Platt

Health authorities hope to vaccinate well over half the population in just a few months against swine flu, which doctors call the 2009 H1N1 strain. That would be a feat. No more than 100 million Americans usually get vaccinated against regular winter flu, and never in such a short period.

How many will race for the vaccine depends partly on confidence in its safety. The last mass inoculations against a different swine flu, in 1976, were marred by reports of a rare paralyzing condition, Guillain-Barré syndrome.

"The recurring question is, 'How do we know it's safe?"' said Dr. Gregory Poland of the Mayo Clinic.

Enter the intense new monitoring. On top of routine vaccine tracking, there are these government-sponsored projects:

• Harvard Medical School scientists are linking large insurance databases that cover up to 50 million people with vaccination registries around the country for real-time checks of whether people see a doctor in the weeks after a flu shot and why. The huge numbers make it possible to quickly compare rates of complaints among the vaccinated and unvaccinated, said the project leader, Dr. Richard Platt, Harvard's population medicine chief.
• Johns Hopkins University will direct emails to at least 100,000 vaccine recipients to track how they're feeling, including the smaller complaints that wouldn't prompt a doctor visit. If anything seems connected, researchers can call to follow up with detailed questions.
• The Centers for Disease Control and Prevention is preparing take-home cards that tell vaccine recipients how to report any suspected side-effects to the nation's Vaccine Adverse Event Reporting system.

"We don't have any reason to expect any unusual problems with this vaccine," said Dr. Neal Halsey, director of Hopkins' Institute for Vaccine Safety, who is directing the email surveillance.

Normal number of reactions

The new H1N1 vaccine is a mere recipe change from the regular winter flu shot that's been used for decades in hundreds of millions of people without serious problems. Nor have there been any red flags in the few thousand people given test doses in studies to determine the right H1N1 dose. They've gotten the same sore arms and occasional headache or fever that's par for a winter flu shot.

But because this H1N1 flu targets the young more than the old, this may be the year that unprecedented numbers of children and pregnant women are vaccinated.

Then there's the glare of the internet — where someone merely declaring on Facebook that he's sure the shot did harm could cause a wave of similar reports. Health authorities will have to tell quickly if there really do seem to be more cases of a particular health problem than usual.

So the CDC is racing to compile a list of what's normal:

• 25,000 heart attacks every week.
• 14,000 to 19,000 miscarriages every week.
• 300 severe allergic reactions called anaphylaxis every week.

Any spike would mean fast checking to see if the vaccine really seems to increase risk and by how much, so health officials could issue appropriate warnings.

1976 concern

Very rare side-effects by definition could come to light only after large-scale inoculations begin — making this the year scientists may finally learn if flu vaccine truly is linked to Guillain-Barré, an often reversible but sometimes fatal paralysis. It's believed to strike between one and two of every 100,000 people. It often occurs right after another infection, such as food poisoning or even influenza.

But the vaccine concern stems from 1976, when 500 cases were reported among the 45 million people vaccinated against that year's swine flu. Scientists never could prove if the vaccine really caused the extra risk. The CDC maintains that if the regular winter flu vaccine is related, the risk is no more than a single case per million vaccinated.

So the question becomes, is the risk of disease greater than that?

Mayo's Poland cites a study in Chicago that found the rate of preschoolers being hospitalized for the new H1N1 flu last spring was 2½ times higher than that possible Guillain-Barré risk.

However the flu season turns out, the extra vaccine tracking promises a lasting impact.

"Part of what we hope is that it will teach us something about how to monitor the safety of all medical products quickly," said Harvard's Platt.