Option 2: Receive an mRNA (Pfizer-BioNTech or Moderna) vaccine for
a second dose
• In line with basic principles of vaccinology, it is expected that combining different
COVID-19 vaccines that induce an immune response against the SARS-CoV-2
spike protein will lead to a robust immune response.
• There is evidence that providing an mRNA vaccine after AstraZeneca COVID-19
vaccine will boost the immune response
o A recent study from Spain (Borobia et al., 2021/CombiVacS) demonstrated
that a heterologous vaccine schedule of an AstraZeneca COVID-19 vaccine
followed by a dose of the Pfizer BioNTech COVID-19 vaccine produces a
strong immune response, as measured by antibodies following the second
dose, when compared to study participants with only a single dose of the
AstraZeneca COVID-19 vaccine.
o In a recent observational study (Barros-Martins et al, 2021), using a ~10.5
week interval between doses, a Pfizer-BioNTech boost following
AstraZeneca COVID-19 vaccine was found to induce a heightened humoral
and T cell immune response compared to the homologous AstraZeneca
COVID-19 vaccine schedule, including an increased neutralizing antibody
response against the Alpha, Beta and Gamma VOCs. The significance of
these findings with regard to effectiveness and duration of protection is
unknown due to a lack of a defined immunological correlate of protection for
SARS-CoV-2 infection.
o Studies involving heterologous schedules with vaccines using different
platforms are ongoing, including the MOSAIC CIRN Canadian study. The
CoM-Cov randomised clinical trial from the United Kingdom (UK), compared
four combinations of the AstraZeneca and Pfizer-BioNTech vaccines at 28
and 84 day intervals. Published results of the 28 day interval comparison
demonstrated a more robust immune response for the
AstraZeneca/COVISHIELD COVID-19 vaccine followed by the Pfizer-
BioNTech vaccine one month following the second dose, compared to a
homologous schedule with the AstraZeneca/COVISHIELD COVID-19 vaccine
(Liu et al., 2021). Data on immunogenicity for the 84 day schedule is
forthcoming. For more information, see the study website:
https://comcovstudy.org.uk/home
• Emerging evidence indicates that heterologous COVID-19 schedules have an
acceptable safety profile. There is direct evidence on the safety of heterologous
COVID-19 immunization schedules (AstraZeneca and Pfizer-BioNTech) from
three studies at dosing intervals between 4 and 12 weeks.
• There is a possibility of increased short-term side effects when using
heterologous COVID-19 vaccine schedules, including headache, fatigue and
myalgia. These side effects are relatively mild, temporary and resolved without
complications.