Safety and immune responses following administration of H1N1 live attenuated influenza vaccine in Thais.

BACKGROUND

Emergence and rapid spread of influenza H1N1 virus prompted health authorities to develop a safe and effective influenza vaccine for domestic use. The Thai Government Pharmaceutical Organization (GPO) with technical support from Russia through WHO had prepared a pandemic live attenuated vaccine (PLAIV) using ca-ts attenuated candidate strain A/17/CA/2009/38 (H1N1) for Thais.

METHODS

Each participant received two doses of intranasal H1N1 vaccine or placebo 21 days apart. All were followed up at 7, 21, 42 and 60 days after first immunization. Blood was drawn for hemagglutination inhibition (HAI) assay from all participants at days 1, 21, 42, and 60 after first immunization. A subset of 40 participants aged 19-49 years was randomly selected for nasal washing at days 1, 21, 42, and 60 to assess IgA using direct enzyme-linked immunosorbent assay (ELISA) along with serum HAI and microneutralization (MN) assay determination.

RESULTS

A total of 363 subjects aged 12-75 years were randomized into 2 groups (271 vaccinees:92 placebos). Almost all AEs were mild to moderate. Local reactions were stuffy nose (22.3%), runny nose (25.1%), scratchy throat (27.2%) and sore throat (19.3%). Systemic reactions included headache (21.7%), myalgia (13.8%), fatigue (16.8%) and postnasal drip (19.9%). On day 60, HAI seroconversion rates for vaccine:placebo group were 30.3:6.0 for ITT and 29.4:5.1 for PP analysis. Children showed highest seroconversion rate at 44, but it decreased to 39.4 when all 3 assays (HAI, MN assay and ELISA) from subgroup analysis were considered.

CONCLUSION

The vaccine candidate is safe. The use of more than one assay may be needed for evaluation of immune response because live attenuated vaccines could effectively induce different kinds of responses. Different individuals could also mount different kinds of immune response, even to the same antigen.