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Monkeypox shows longevity of smallpox shots

A 2003 outbreak of monkeypox in the United States has helped prove that smallpox vaccinations can protect for decades, U.S. researchers said.
/ Source: Reuters

A 2003 outbreak of monkeypox in the United States has helped prove that smallpox vaccinations can protect for decades, U.S. researchers said Monday.

The study could help officials trying to come up with a plan for mass vaccination against the often deadly smallpox virus and its relatives, should such a virus ever be used in a biological attack.

The researchers found three people who were evidently infected by the monkeypox virus, spread mostly by pet prairie dogs in the Midwest, who never had any symptoms.

The three had last been vaccinated against smallpox before the jabs were discontinued. One was vaccinated 13 years ago, another 29 years ago and the third 48 years ago.

“These individuals were unaware that they had been infected because they were spared any recognizable disease symptoms,” Mark Kenneth Slifka of Oregon Health & Sciences University and colleagues wrote in their report, published in the journal Nature Medicine.

“Together, this shows that the U.S. monkeypox outbreak was larger than previously realized and, more importantly, shows that cross-protective antiviral immunity against West African monkeypox can potentially be maintained for decades after smallpox vaccination.”

Smallpox was eradicated in 1979 after a global vaccination program. But experts fear the virus or a related virus could be used as a biological weapon.

About half the U.S. adult population has been vaccinated as part of routine efforts that stopped in 1972 for civilians and 1990 for the military. Studies have shown that immunity lasts as long as 75 years.

Now work is underway to stockpile doses of vaccine in case of an attack, while tests are also underway to see if perhaps some people might be protected by their old vaccinations.

But no one has been able to do a real test of how well the vaccine protects -- until the monkeypox outbreak, traced to pet rodents imported from West Africa, where the close relative of smallpox is well entrenched and still common.

The team tested 44 people from Wisconsin, the epicenter of the outbreak, which originated at a pet store in Illinois. At least 39 people were reported infected with monkeypox in Wisconsin, they said.

“We screened subjects and included in the study only those who had close contact with monkeypox-infected individuals or monkeypox-infected prairie dogs,” they wrote.

They found three people who had clear evidence of infection in their blood, yet never showed any symptoms of monkeypox. All had been previously vaccinated against smallpox.

Spread without direct contact
They also found that the pox viruses spread even without direct contact -- something to be aware of should there ever be a smallpox attack.

“A common misconception of the U.S. monkeypox outbreak is that infection requires direct contact or direct inoculation through scratches or bites in order for infected prairie dogs to transmit monkeypox to humans,” the researchers wrote.

But the virus apparently can be carried in the air and by tiny sneezed droplets.

“In one case, a subject contracted monkeypox after an infected prairie dog was carried into her home when she was not present. The prairie dog was apparently not placed on the floor or furniture, and yet this subject, who had no contact with prairie dogs or other individuals with monkeypox, contracted the disease,” they wrote.