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Most People Given Smallpox Vaccine in Past Probably Still Immune


NEW YORK (Reuters Health) Aug 18 - More than 90% of people who
received the smallpox vaccine 25 to 75 years ago show substantial
immunity against vaccinia, the virus used in the vaccine, according to
a report published in the August 17th online issue of Nature Medicine.

Dr. Mark K. Slifka, from Oregon Health & Science University in
Beaverton, and colleagues evaluated the degree and duration of
vaccinia immunity after one or more smallpox vaccinations.

Even decades after vaccination, most people showed significant
humoral, cellular, or combined immunity against vaccinia. Antiviral
T-cell responses declined slowly over time, but the antibody responses
remained stable for up to 75 years.

"If these levels of immunity are considered to be at least partially
protective, then the morbidity and mortality associated with an
intentional smallpox outbreak would be substantially reduced," the
researchers state.

Although this is encouraging news, there are still many individuals
born after the mid-1970s who never received the smallpox vaccine. In
the event of a bioweapon attack with smallpox, these people would be
completely vulnerable to the viral pathogen.

Dryvax, the vaccine that eradicated smallpox as a natural disease, was
created by growing the viral agent in the skin of calves or other
large animals, a production method that is no longer acceptable today.
Moreover, there is only enough Dryvax left to vaccinate a few million
people, ideally health workers and first responders.

One proposed solution to this vaccine shortage has been to expand the
Dryvax stocks through dilution. Unfortunately, even if vaccine
efficacy was not compromised, there would still not be enough vaccine
to cover everyone.

Another study in Nature Medicine, however, may provide some good news.

Dr. Richard Weltzin, from Acambis, Inc. in Cambridge, Massachusetts,
and colleagues have developed several vaccine candidates derived from
the existing Dryvax stocks. Specifically, the researchers were
interested in a vaccine that could be manufactured in cell culture in
large amounts.

One of the candidates, a clone designated ACAM1000, performed similar
to Dryvax in animal studies, so the researchers decided to compare the
two in a randomized human study.

In the clinical trial, ACAM1000 was just as effective as Dryvax at
producing major cutaneous reactions and at inducing humoral and
cell-mediated immunity against vaccinia virus. The two vaccines also
demonstrated equivalent safety profiles.

In some ways, however, ACAM1000 may actually be better than Dryvax.
ACAM1000 "probably represents an advance over Dryvax in terms of
purity, quality, freedom from bacterial contamination allowed in the
old vaccine, the absence of bovine adventitious agents and possible a
lower incidence of postvaccinal encephalitis," the authors note.

Nat Med 2003;August 17 online issue:000-000.

http://www.medscape.com/viewarticle/460223?WebLogicSession=P0E4VmhVyzhO3LfPbjcqAvVFh34 pfzDY7UJmBNdi0OVn0Bdo04gh|6818469695563490190/184161395/6/7001/7001/7002/7002/7001/-1