Table 2.
Incidence of and vaccine efficacy against laboratory-confirmed influenza among children participating in a trial of trivalent inactivated influenza vaccine (IIV3) in Dhaka, Bangladesh, 2010–2014.
| IIV3 |
IPV |
Vaccine efficacy (95% CI)a | P-valueb | |||||
|---|---|---|---|---|---|---|---|---|
| N events | Child-years at risk | Incidence (per 100 child-years) | N events | Child-years at risk | Incidence (per 100 child-years) | |||
| All influenzac | 200 | 2,009 | 10.0 | 294 | 2,034 | 14.5 | 31.1 (17.5, 42.4) | <.001 |
| 2010 Seasond | 1 | 219 | 0.5 | 1 | 219 | 0.5 | −0.2 (−1501, 93.7) | >.99 |
| 2011 Season | 62 | 646 | 9.6 | 112 | 651 | 17.2 | 44.2 (24.0, 59.1) | <.001 |
| 2012 Season | 41 | 599 | 6.8 | 49 | 621 | 7.9 | 13.3 (−31.3, 42.7) | .5 |
| 2013 Season | 96 | 545 | 17.6 | 132 | 544 | 24.3 | 27.4 (5.6, 44.2) | .017 |
| Influenza virus subtype/lineage | ||||||||
| Influenza A (H1N1pdm09) | 25 | 2,011 | 1.2 | 38 | 2,039 | 1.9 | 33.3 (−10.5, 59.7) | .116 |
| Influenza A (H3N2) | 118 | 2,010 | 5.9 | 173 | 2,036 | 8.5 | 30.9 (12.7, 45.3) | .002 |
| Influenza B/Victoria | 40 | 2,011 | 2.0 | 61 | 2,038 | 3.0 | 33.5 (1.0, 55.4) | .045 |
| Influenza B/Yamagata | 13 | 2,012 | 0.6 | 18 | 2,039 | 0.9 | 26.8 (−49.4, 64.1) | .391 |
| Vaccine-matched influenza virus infectionse | ||||||||
| Overall | 138 | 2,010 | 6.9 | 229 | 2,037 | 11.2 | 38.9 (24.6, 50.6) | <.001 |
| 2010 Seasond | 1 | 219 | 0.5 | 1 | 219 | 0.5 | −0.2 (−1501, 93.7) | >.99 |
| 2011 Season | 51 | 646 | 7.9 | 98 | 652 | 15.0 | 47.5 (26.4, 62.6) | <.001 |
| 2012 Season | 11 | 599 | 1.8 | 23 | 621 | 3.7 | 50.4 (−1.7, 75.8) | .055 |
| 2013 Season | 75 | 546 | 13.7 | 107 | 545 | 19.6 | 30.0 (6.0, 47.9) | .018 |
IIV3 = Trivalent Inactivated Influenza Vaccine; IPV = Inactivated Polio Vaccine.
P-values compare IIV3 to IPV using unadjusted Poisson regression.
Vaccine efficacy estimated as 100% × (1 − rate ratio).
Influenza virus found in nasopharyngeal wash using RT-PCR.
The 2010 season occurred from September 2010 to March 2011, 2011 season occurred from April 2011 to March 2012, 2012 season occurred from April 2012 to March 2013, and 2013 season occurred from April 2013 to March 2014.
Influenza viruses from trial participants were antigenically characterized and compared with trivalent inactivated influenza vaccine antigen using hemagglutinin inhibition (HAI) assay. Viral isolates were considered similar to the vaccine (i.e. vaccine-matched) if antisera raised against the vaccine virus antigen reacted with the isolate with an HAI titer that was at least 4-fold higher than the reaction to other reference antisera.