30-Year-Old Smallpox Vaccinations May be Useless

People who were inoculated against smallpox more than 30 years ago may have lost their immunity against the deadly virus, the preliminary results of a new study suggest.

Researchers at St. Louis University in Missouri tested 90 people who'd been vaccinated as children to see if diluted boosters of the vaccine would produce an immune response, the St. Louis Post-Dispatch reports. If the vaccine was no longer in their systems, the booster shots would leave a puss-filled sore on the skin that would scab over and heal, "leaving a circular mark and about three to five years' worth of protection against smallpox and its relatives."

Dr. Sharon Frey, the lead researcher, said the subjects reacted as though they'd never been inoculated before. They all developed the tell-tale spherical scar, regardless of whether they received undiluted or diluted vaccine.

The researchers are taking their study nationwide. They need more than 900 volunteers to determine how much vaccine to give people who were vaccinated long ago.

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-- Felicity Stone