ACIP recommends against mass smallpox vaccination for the public

July 2002

ATLANTA — The Advisory Committee on Immunization Practices (ACIP) voted to recommend limited use of the smallpox vaccine for preexposure vaccination and will not recommend mass vaccination of the general public.

Information provided to the panel indicated that the known risk of a smallpox release is low; therefore, the risk of adverse events after vaccination does not outweigh the potential benefit of widespread preexposure vaccination. According to evidence presented to the committee, in the absence of screening, widespread vaccination could result in 200 serious adverse events – including eczema vaccinatum, progressive vaccinatum, encephalitis and even death – for every one million vaccinees.

Who should be vaccinated?

The ACIP did recommend that every state vaccinate smallpox response teams, including medical and law enforcement personnel who would respond to a smallpox incident. States should also designate smallpox health care teams at predetermined hospitals where smallpox cases would be referred.

If a smallpox case were detected in the United States, the CDC would respond with surveillance and containment, including ring vaccination, the same strategy used by public health officials to eradicate smallpox in the 1970s.

Before eradication, transmission of the virus was slow and did not occur in the prodromal stage of the illness. Infected individuals were generally not ambulatory and were usually ill themselves limiting transmissibility. As well, the greatest risk of transmission occurred among household contacts with frequent and sustained exposure. Epidemiologic evidence suggests that smallpox has a lower rate of transmission than diseases such as measles, pertussis or influenza.

However, smallpox virus can be shed by vaccinees and spread to others. In prior experience, this would enact herd immunity for the general public, but more people today have immuno-deficiencies, including HIV, who could suffer significant complications and perpetuate the virus in the general public. The ACIP reviewed several algorithms for prescreening potential vaccine candidates for contraindications and any final smallpox plan will contain stipulations for screening any individual who could potentially suffer complications from vaccination, they said.

The recommendations contain a provision for continuous monitoring of the threat of smallpox release. If the panel determines that the risk changes, preexposure vaccination could be expanded and the CDC can designate a plan consistent with need. The smallpox plan also assumes that health officials can quickly disseminate smallpox vaccine if needed.

There is no licensed smallpox vaccine available in the United States, but the CDC does have control of about 15 million doses of Dryvax (Wyeth) smallpox vaccine and another 85 million doses of a smallpox vaccine donated by Aventis Pasteur. Studies have shown that the Wyeth product can be diluted up to five times, meaning the government could vaccinate over 150 million people if a smallpox case was detected.

Last November, the government contracted with Acambis and Baxter to produce another 209 million doses of smallpox vaccine. Unlike the Wyeth and Aventis products, which were produced using calf lymph tissue using a seed virus derived from the New York City Board of Health strain of vaccinia virus, the Acambis product will be made using cell culture technology.

The smallpox recommendations from the ACIP are meant to update its current policy on preexposure vaccination, which states that only laboratory workers who handle non-highly attenuated orthopox viruses should be vaccinated. The recommendations will be presented to the CDC and the Department of Health and Human Services for final review.