The Global Advisory Committee on Vaccine Safety (GACVS) held its eighth meeting in Geneva, Switzerland, on 11-12 June, 2003, and considered, inter alia, the following safety concerns regarding immunization of immunocompromised individuals, with special reference to BCG vaccination: adverse events associated with intranasal administration of vaccines; theoretical risks associated with vaccines that might become contaminated with the agent responsible for transmissible spongiform encephalopathy and the actions that have been taken to date by WHO in anticipating and preventing these risks; safety of vaccines containing thiomersal and the ongoing efforts of WHO in monitoring the situation; the safety of yellow fever vaccine and the risks and benefits of repeat vaccination, including for international travel; lack of evidence linking hepatitis B vaccination and childhood leukaemia; information sharing regarding vaccine safety between manufacturers, regulatory agencies and the public; safety of smallpox vaccines in general; adverse events following mumps vaccination, with special reference to the risk of aseptic meningitis; safety of aluminium-containing vaccines and alleged associations of these vaccines with macrophagic myofasciitis and chronic fatigue syndrome; and an update on the potential adverse impact of routine vaccination on child survival.
Regarding child survival following immunization, GACVS concluded that no further information is available to support the alleged increased risk of nonspecific mortality following childhood vaccination and affirmed its previous conclusion that the evidence does not support the suggested link(1). Regarding the safety of aluminium-containing vaccines, the Committee concluded that no additional data have emerged that would result in a modification of its previous statements regarding macrophagic myofasciitis and the use of aluminium-containing vaccines. It also concluded, with respect to chronic fatigue syndrome, that there are no data to support a causal relationship with aluminium-containing vaccines. GACVS reiterates its previous statements that there should be no change in current recommendations for the use of aluminium-containing vaccines, including their intramuscular administration(1,2).
Other conclusions of GACVS taken at its most recent meeting include the following.
Adverse events following mumps vaccination
GACVS considered a comprehensive review of the world literature regarding the safety of mumps vaccination, with special attention being paid to the risk of vaccine-derived mumps meningitis. It was noted that higher rates of aseptic meningitis have been described for the Urabe, Leningrad-Zagreb and Leningrad-3 strain vaccines compared with the Jeryl-Lynn strain vaccine. The possible virological basis for this difference and/or the other characteristics of the product that might explain these differences are not known. Some of the variability observed in the risk of aseptic meningitis following use of the various mumps vaccine strains may reflect pre-existing immunity, in particular in older age groups, as well as the variable levels of sensitivity of surveillance and of diagnostic practices in different settings. A detailed final report of this review, in which the estimate of risks will be assessed, will be published in due course.
GACVS concluded that risk estimates vary between studies, reflecting differences in study settings and circumstances and in degrees of surveillance. The available data are insufficient to distinguish between the safety profile with regard to aseptic meningitis for Urabe, Leningrad-Zagreb and Leningrad-3 strains. The Committee is not aware of any cases of virologically proven aseptic meningitis following Jeryl-Lynn vaccine. If Urabe, Leningrad-Zagreb and Leningrad-3 strain vaccines are being used in mass vaccination campaigns, national immunization programmes need to take into account the potential for clustering of aseptic meningitis following the campaigns. The Committee noted that, until now, all reported cases of vaccine derived mumps meningitis have recovered. There is no known case with long-term sequelae.
GACVS further considered a proposal for a mumps vaccine virus strain bank, to be developed at the invitation of WHO, which has considerable potential scientific interest. WHO will give further attention to the possibility of such a strain bank being established.
Safety of smallpox vaccines
Two expert reports on the safety of smallpox vaccines were considered in detail. Special attention was paid to the paucity of data regarding safety of immunization in subjects < 18 years of age, age-related risks with the vaccine in general and outcomes in women immunized during pregnancy. Current data are insufficient to define the incidence of adverse events in primary vaccinees as opposed to individuals revaccinated after a long interval. The Committee noted the importance of adverse event surveillance programmes being open-minded so that hitherto unrecognized events might be detected. If the vaccine is being used in mass campaigns, it would be especially important for smallpox immunization programmes to be supported by adverse event monitoring. This applies particularly to countries with a high prevalence of HIV infection.
GACVS concluded that there is a real risk of serious adverse events following immunization with smallpox vaccine, including safety issues that have not previously been recognized, that there may be potential risks to contacts of vaccinees and that implementation of immunization would require significant capacity and resources. GACVS will continue to monitor the safety of smallpox vaccines.
Safety of BCG vaccination in immunocompromised individuals
GACVS noted that there has been repeated reference to local or disseminated BCG infection several years after BCG immunization in HIV-positive persons. There needs to be closer monitoring of these adverse events in areas of high HIV prevalence, with specific efforts to distinguish BCG infection from tuberculosis. Currently, no change in vaccination policy is recommended, although the risk-benefit relationship should be continually assessed, and surveillance of HIV-positive persons who receive BCG vaccines should be continued for at least 5 to 7 years. In the development of new live attenuated vaccines against tuberculosis, account needs to be taken of the special safety issues for immunocompromised recipients.
GACVS is maintaining a watching brief on the safety of thiomersal-containing vaccines(3). There is insufficient evidence to reach definite conclusions regarding the safety of thiomersal in possible special risk groups, notably malnourished infants and premature or low-birth-weight newborn infants. It is important to determine whether such individuals are at special risk, and WHO should encourage further research on the matter relevant to the developing world. Based on the most recent evidence, GACVS reported to WHO that there is no scientific basis for changing current WHO recommendations for thiomersal-containing vaccines, including administration of a birth dose of hepatitis B vaccine and vaccination of low-birth-weight infants where indicated.
Global Advisory Committee on Vaccine Safety, 20-21 June, 2002. WER 2002;47:389-404.
Vaccine safety. WER 1999;41:337-40.
GACVS Web site. URL: <http://www.who.int/vaccine_safety/en/>.
Source: WHO Weekly Epidemiological Report, Vol 78, No 32, 2003