Influenza Outbreak in a Medical College in Palakkad, Kerala, India, September–October 2021

Background: In September–October 2021, Palakkad district, Kerala, faced an H3N2 influenza outbreak among medical and paramedical students. Objectives: We described the outbreak and determined the agent, source, and risk factors. Materials and Methods: To investigate the outbreak, we conducted an active case search using an online questionnaire. We defined a case as fever with any of the associated symptoms of cough, running nose, headache, sore throat, tiredness, or breathlessness. We described cases by time, place, person, and attack rates (ARs), and adjusted risk ratios (aRRs) were calculated. Results: Among 780 students, 607 responded, with 122 cases of H3N2 (20.1%). Higher ARs were observed among females (23%), MBBS students (24%), and ladies’ hostel inmates (22%). We conducted a retrospective cohort study among all the students to determine the risk factors. Univariate analysis identified MBBS students, residing in the ladies’ hostel, improper mask use and having close contact at a common place as risk factors for being a case. Multivariable analysis confirmed improper mask use (aRR-2.2 [95% confidence interval (CI):1.4–2.7]) and residing in ladies’ hostels (aRR-2.6 [95%CI: 1.6–4.4]) as significant risk factors, supported by environmental factors like poor ventilation. Conclusion: The outbreak was caused by the H3N2 influenza virus. Residing in the ladies’ hostel, poor compliance with mask-wearing and close contact during gatherings were identified as risk factors for the outbreak. We recommended proper mask use among students as well as isolation of cases. Swift public health action contained the outbreak by November 2, 2021.