The preliminary results were released as a pre-print study in The Lancet in September and have not yet been peer-reviewed.
Covid-19 booster vaccines are also safe to receive alongside the flu vaccine.
In the new study's trials, doctors recruited 679 volunteers from April to June at 12 sites in Britain.
During the study, all volunteers had received the first dose of a Covid-19 vaccine from either Pfizer-BioNTech or AstraZeneca, per the New York Times.
Half of the participants received a flu shot when the groups returned for their second dose of the Covid-19 vaccine, while the other half received a placebo.
After receiving their doses of the vaccines, the research team monitored the study groups for fevers or aches.
Previously, the Centers for Disease Control and Prevention (CDC) recommended spacing out Covid-19 and flu vaccines by 14 days. »