But scientists say any short-term protection against infection is likely to fall away quickly.
The study of 133 people, two weeks after their fourth dose, found the vaccines were "well-tolerated" and "boosted immunity".
All participants, some of whom were over 70 and some under 70, were vaccinated with a dose of Pfizer or a half dose of Moderna about six months after having their third dose. Another part of the immune system called T-cells were also boosted after the fourth dose, which suggests longer-lasting protection against severe disease was increased.
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