In January 1997, a proposal on asthma studies was put forward by a researcher at the Centers for Disease Control and Prevention (CDC), and a working group consisting of interested investigators from the VSD, managed care organizations (MCOs), and CDC was established to conduct such studies.1 In fall 1997, a paper by Kemp and colleagues (Kemp et al., 1997) suggested an association between whole-cell pertussis vaccine and asthma. Around that time, several articles were published on the hygiene hypothesis (Mawson, 2001), which posits that some infections in infancy or early childhood may protect against asthma or other allergic conditions. That raised the idea that vaccines, by preventing the infections, may increase the risk of asthma. The National Immunization Program (NIP) decided that this topic should be addressed by using VSD data, so NIP-affiliated VSD researchers conducted a pilot study in 1998 (DeStefano, 2004).
The study team was able to conduct data-quality and data-assurance checks of the electronic data and perform chart review on a sample of about 5% of the VSD population. The study team used that 5% sample to do the analyses. The analyses were presented at a meeting of VSD investigators in June 1998 and at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in September 1998. The researchers determined that even though they had reviewed the patient charts, there was uncertainty about the MCO enrollment status of children; some were enrolled from birth, and others were enrolled when they were 2 or 3 years old. The researchers could not determine with certainty the patients’ vaccine histories before they were enrolled in the MCO. From the pilot study, the researchers learned that the data were not complete enough to be sure that the children who did not have vaccinations in their records were not vaccinated (DeStefano, 2004).
The researchers revised their study on the basis of what they had learned and restricted the data analysis to children born as MCO members to obtain a complete vaccination history. The initial findings from the new analysis were presented at the May 1999 meeting of VSD investigators. There was still some concern about the validity of vaccination status, and to account for it, the researchers conducted a subanalysis of children who had at least some indication of using the MCO for health care. The findings of the analysis were presented at the International Conference on Pharmacoepidemiology in August 1999 and at the September 1999