Licensed influenza vaccines are manufactured with a variety of technologies. The majority are split, inactivated vaccines derived from egg-adapted, high growth reassortant viruses. Two US licensed products do not use egg-adapted viruses: Flucelvax (mammalian cell culture) and FluBlok (recombinant). There is increasing evidence that egg propagation induces virus mutations that impair the immune responses to circulating viruses. However, the impact of egg-propagation on clinical vaccine effectiveness is uncertain, and there is no preferential recommendation for any specific influenza vaccine product or technology. A direct comparison of serologic response to egg based and non-egg based vaccines in adults has not been performed. This randomized trial will compare serologic responses to the egg- and non-egg A(H3N2) vaccine component. The study cohort will be followed for two influenza seasons to evaluate sequential vaccination effects on immune response.

Inclusion Criteria:

• Aged 18-64 years
• Marshfield Clinic patients living in or around Marshfield, Wisconsin since July 2015
• Willing and able to give informed consent and comply with study requirements

Exclusion Criteria:

• Receipt of 2018-19 influenza vaccine prior to study enrollment
• Known to be pregnant at the time of enrollment
• Current participation or plans to participate in another clinical trial involving an experimental agent
• Presence of a contraindication to influenza vaccination
• Plans to relocate outside the geographic location in the next two years