While both heterologous (mixing) and homologous (matching) vaccine regimens are now considered standard of care, post-vaccination complications and long-term effects of the different vaccination regimens have not been thoroughly studied. There is a pressing need to investigate the longitudinal effects of the mixing and matching vaccine-booster approaches. This study proposes to utilize the existing digital infrastructure of the COVID-19 Citizen Science (CCS) study on the Eureka Research Platform to perform a systematic and prospective randomized trial comparing mixing versus matching approaches. Eligible CCS participants will have the opportunity to be randomly assigned to a recommendation of receiving either the Pfizer or Moderna booster vaccine. Long-term effects will be monitored through the participants' completion of their regular weekly CCS follow-up surveys on symptoms and infection. This randomized trial aims to mitigate the effect of confounding variables and provide more conclusive evidence on each regiment to guide booster recommendations.

The FDA recently approved the use of COVID-19 booster vaccines for both homologous and heterologous vaccine booster regimens, with both regimens considered standard of care. However, there is incomplete research on the long-term effects and immunity offered by each approach. In fact, the FDA has not offered guidance on whether or not to mix or match vaccines because as of yet, there is no evidence indicating that one strategy should be recommended over the other. Studies have shown that vaccine efficacy declines over time, and the Delta and Omicron variants continue to cause breakthrough cases. Additionally, millions of Americans are now becoming eligible for a booster. These factors present both a pressing need and a unique opportunity to investigate the longitudinal effects of the mixing and matching vaccine-booster approaches.

This study proposes to fill this critical gap through a systematic and prospective randomized trial utilizing the existing digital infrastructure of the COVID-19 Citizen Science (CCS) study on the Eureka Research Platform. Eligible CCS participants will have the opportunity to be randomly assigned to be encouraged to receive either the BNT162b2 (Pfizer) or mRNA-1273 (Moderna) booster vaccine and will continue to complete their weekly CCS follow up surveys on symptoms and infection. Thus, this study will allow for longitudinal follow up of participants receiving homologous and heterologous vaccine booster regimens. By randomizing participants and analyzing a larger cohort, this study aims to mitigate the effect of confounding variables and provide more conclusive evidence to guide booster recommendations.

Inclusion Criteria:

• Must be enrolled in the COVID19 Citizen Science (CCS) study. The CCS study is a remote, longitudinal, observational registry hosted on the Eureka Research Platform that administers weekly questionnaires to participants regarding COVID-19 symptoms and exposures. CCS is currently open to enrollment for adults aged 18 and older who have a smartphone device to participate on the mobile app, or an email address to participate on the web portal.
• Must have received either 2 doses of mRNA vaccine such as Moderna and Pfizer, or 1 dose of Johnson and Johnson

Exclusion Criteria:

• Already received a COVID19 booster
• Are not planning on receiving a COVID19 booster in the next 3 months
• Do not have equal access to both the Pfizer and Moderna boosters
• Have a medical reason to receive either the Pfizer or Moderna booster
• Have a medical reason to not receive any booster (e.g h/o allergic reaction to an mRNA vaccine or myocarditis or pericarditis after an mRNA vaccine)
• Are unwilling to be randomized
• Are unable to commit to filling out the CCS weekly surveys consistently (to the best of their ability)