Seven studies with 10 trials (5117 subjects) met the inclusion criteria for the meta-analysis for field studies. Twelve studies with 52 trials (12,341 subjects) met the inclusion criteria for the meta-analysis for serologic studies.
The pooled protection-rate difference in field studies was close to 0 (1.1%, 95% CI: -0.2%, 2.4% random-effects model), thus detecting no difference between single or multiple vaccination.
In the serologic studies, 9 trials were significantly in favour of single vaccination, and 7 trials were in favour of multiple vaccination, but in most cases, there was no significant difference between the two vaccination groups. The pooled serologic protection-rate difference from 52 trials was close to 0 (1.7%, 95% CI: -1.31%, 4.8% random-effects model), thus detecting no difference between single or multiple vaccination. Subdividing according to age, study design, H1 assay methods or vaccine types did not change the result.