A single-blind, randomised, phase II UK multi-centre study to determine reactogenicity and immunogenicity of heterologous prime/boost COVID-19 vaccine schedules (Com-COV1)

Project summary:

This is a single-blind randomised trial studying combinations of two different vaccines. The purpose of the trial is to determine whether having doses of two different types of COVID-19 vaccines gives as good an immune system response as having two doses of the same vaccine and to see how good the response is when the second dose is separated from the first dose by different periods of time. Adults aged 50 years and above were allocated at random to receive one dose of one approved vaccine and a second dose of either the same approved vaccine, or a dose of a different approved vaccine. Participants were also allocated at random to the timing of receiving their second doses (boost) – either at four weeks or at twelve weeks after the first dose (prime). The vaccines offered through the trial were AstraZeneca and Pfizer.

Participants were asked to complete an electronic diary for up to three months after the second (boost) vaccine in order to record any symptoms or medical conditions.

The last visit (D364) of the trial has been brought forward to accommodate the third dose currently offered by NHS and all study visits ended January 2022.



The UK Vaccine Task Force and the National Institute for Health Research (NIHR)

Leader researcher:

Professor Matthew Snape

Lead institution:

University of Oxford

Vaccine type:


Recruitment for clinical trial open:



COM-COV website