The symbol ∑ x , n t , x ̂ , n ̂ t M N t is defined as a fourfold summation, two for all x , x ̂ ∈ X and two for the indices n t , n ̂ t ∈ 1, …, N t.
Seroconversion was defined as a fourfold increase in HI titer or from baseline (≤8) to HI titers ε32.
SCII was defined as a fourfold or greater titre rise between pre- and post-season samples, with a postseason titre of at least 40.
Seroconversion (defined as a fourfold rise in end-point titers in acute- and convalescent-phase samples) was demonstrated in seven case-patients.
Treatment-emergent immunogenicity was defined as a fourfold increase from baseline titer or an increase of at least two dilutions during the treatment or follow-up period.
Seroconversion to E. chaffeensis was defined as a fourfold or greater increase in antibody titer between acute-phase or preseason and convalescent-phase samples.
A human case was defined as a fourfold change in serial F. tularensis antibody titers from serum samples obtained at least 14 days apart, with at least one titer ≥1 128, in an exposed person.
In a cohort of 468 patients with P. aeruginosa infections, 30 patients developed resistance during treatment, defined by a fourfold increase in minimum inhibitory concentration (MIC) relative to the baseline isolate, which resulted in a change in interpretive class [ 25].
The presence of an anamnestic response was defined as a fourfold or greater rise in anti-HBs titers between pre- and post-booster administration, providing that a titer ≥ 10 mIU/ml was reached after the booster.
The main outcome measure was serologically confirmed influenza infection (SCII), defined as a fourfold or greater rise in haemagglutination inhibition antibody titres to a circulating strain of influenza (with post-season titre at least 1 40).
