Right sizing for vaccine effectiveness studies: how many is enough for reliable estimation?

Background The precision of vaccine effectiveness (VE) estimates is dependent on sample size and sampling methods. In Victoria, participating general practitioners (GPs) are not limited by the number of influenza-like illness (ILI) patients they collect respiratory samples (swabs) from in sentinel surveillance. However, in the context of scarce resources it is of interest to determine the minimum sample size needed for reliable estimates. Methods Following the test-negative design, patients with ILI were recruited by GPs and tested for influenza. Descriptive analyses were conducted to assess possible selection bias introduced by GPs. VE was calculated by logistic regression as [1 – odds ratio] x 100% and adjusted for week of presentation and age. Random 20% and 50% samples were selected without replacement to estimate the effect of swab rates on VE estimates. Results GPs swabbed a smaller proportion of patients aged ≥65 years (45.9%, n=238) than those <5 (75.6%, n=288), 5–17 (67.9%, n=547) and 18–64 (75.6%, n=2662) years. Decreasing the swab rate did not alter VE point estimates significantly. However, it reduced the precision of estimates and in some instances resulted in too small a sample size to estimate VE. Conclusion Imposing a 20% or 50% swabbing rate produces less robust VE estimates. The number of swabs required per year to produce precise estimates should be dictated by seasonal severity, rather than an arbitrary rate. It would be beneficial for GPs to swab patients systematically by age group to ensure there are sufficient data to investigate VE against a particular subtype in a given age group.

EFFECTIVE SIZE OF AUTO-TETRAPLOID POPULATION UNDER PARTIAL SELFING

Effective population size is defined as the number of breeding individual in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration. Effective population size depends on the census size of the population and the mating system. In autotetraploid population, effective population size also depends on the probability of double reduction, i.e., a meiotic event when two sister chromatids end in the same gamete. In this research, we will study the effect of the probability of double reduction on the effective size of autotetraploid population reproduced by partial selfing. The formula for the effective population size was derived by equating the variance of the change in gene frequency in idealized population and its value in the autotetraploid population with arbitrary rate of partial selfing and double reduction. The resulted formula, and numerical study based on the formula, indicated that the effective size decreases by the increase of probability of double reduction and the rate of selfing. When there is complete selfing, however, the effective size is not affected by the probability of double reduction.