Interim or mid-season estimates of flu vaccine effectiveness for the 2019-20 season were published Thursday in the Morbidity and Mortality Weekly Report (MMWR) by the Centers for Disease Control and Prevention (CDC). Estimates were based on 4,112 enrollments at Marshfield Clinic Health System and four other U.S. sites. The Health System contributed over 800 enrollments to the analysis.

Mid-season results are encouraging

Vaccination reduced the risk of flu by nearly half (45%). This is similar to previous seasons when circulating flu viruses were similar to the vaccine viruses. This season the highest level of protection (>50%) is seen in children. Vaccine effectiveness against A/H1N1pdm09 was somewhat lower (37%) compared to prior seasons. The explanation for this is not known and additional investigations are planned.

The majority of flu infections were caused by B/Victoria viruses, with increasing levels of A/H1N1pdm09 in recent weeks. Both flu A and B viruses can cause severe illness. In Wisconsin, there have been three pediatric deaths and over 1,900 hospital admissions so far this season, according to the Wisconsin Department of Health Services. 

“Children have been hit hard this season, and providers should be strongly recommending vaccination to parents of unvaccinated children," said Edward Belongia, M.D., principal investigator for the Marshfield flu study and Center for Clinical Epidemiology and Population Health Director. “Vaccination should continue as long as flu viruses are circulating."

Research coordinators are actively recruiting patients presenting with cough, cold, or flu-like symptoms for multiple flu studies in primary and urgent care departments at Chippewa Falls, Eau Claire, Lake Hallie, Marshfield, Rice Lake, Wausau and Weston centers. Many of these departments have been busy with a high volume of patients this season.

“As always, providers and clinical staff have been amazing, graciously accommodating our research staff during this hectic time," said, Deanna Cole, research operations manager. 

Nasal and throat swabs obtained from eligible and consenting patients are tested for flu by the Research Institute's Integrated Research and Development Laboratory. As of mid-February, over 1,600 patients have been enrolled in the Marshfield CDC flu study and 542 were positive for influenza; 57% of positives are B/Victoria and 31% are A/H1N1pdm09. Community support for the study continues to be high with a participation rate close to 80%.