Cumulative Incidence Estimates of Medically Attended Seasonal Influenza from 2011-2016 for the Central Texas Baylor Scott & White Health - Temple Population Research Area (BSWH-TPRA).

Background: Severity and burden of influenza can be estimated with the cumulative incidence of lab-confirmed influenza among medically-attended acute respiratory illnesses (MAARI) during local circulation.

Methods: BSWH is one of 5 CDC US Flu Vaccine Effectiveness network sites since 2011. The BSWH-TPRA source population was defined as residents from zip codes within East Bell County, Central Texas (CTX), who have seen a primary care provider for any reason in the past 3 years. We enrolled outpatients with acute respiratory illness of ≤7 days, with cough and tested nasal-throat swabs for influenza A&B by RT-PCR. Vaccination was verified with electronic records. We divided the population into 4 age strata and 2 vaccination statuses. Within each stratum and status, we formed 3 subgroups based on number of MAARIs: (None, 1, and ≥2). Applying proportions observed in the enrollees, we estimated the cumulative incidence of medically-attended seasonal influenza.

Results:

Table: Cumulative Incidence of Seasonal Influenza in the BSWH-TPRA 2011-12 to 2015-16.

Influenza Season (Severity)	Percentages of predominant Influenza Type, A Subtype and B lineage in enrollees	Cumulative Incidence % (95% CI)
2011-12 (Mild)	A (H1N1)pdm09 (84%)	0.72% (0.53-0.92)
2012-13 (Moderately-severe)	A (H3N2) (51%), B-Yamagata (36%), B-Victoria (10%)	5.95% (5.52-6.38)
2013-14 (Moderate)	pH1N1 (77%)	2.44% (2.12-2.77)
2014-15 (Moderately-severe)	A (H3N2) (75%)	4.14% (3.73-4.55)
2015-16 (Moderate)	A (H1N1)pdm09 (27%), B-Victoria (46%)	2.61% (2.23-2.99)

The highest incidence during 2011-12, 2012-13 and 2014-15 was among persons ages 6 months - 8 years, 1.61%; 95% CI (0.88-2.33%), 8.96%; 95% CI (7.48-10.43%) and 7.27%; 95% CI (5.83-8.72%) respectively. During 2013-14, it was among those ages ≥50 years: 2.79%; 95% CI (2.17-3.42%) and during 2015-16, it was seen among those 9 years -18 years: 5.05%; 95% CI (3.91-6.19%).

Conclusion: The cumulative incidence of medically-attended influenza in 2013-14, when the burden of A (H1N1)pdm09 shifted to those aged ≥50 years was higher than in 2011-12. The highest incidence was seen in 2012-13 season when A H3N2 and both B-lineages circulated. Influenza immunization programs should especially target age-groups 6 months to 18 years and ≥50 years.