Hybrid immunity and protection against infection during the Omicron wave
in Malta
Abstract
Background: By December 2021, administration of the third dose of
COVID-19 vaccinations coincided with the spread of the Omicron variant
in Europe. Questions had been raised on protection against infection
conferred by previous vaccination and/or infection. Method: Our study
population (n = 252,433) included all those in the COVID-19 vaccination
registry in Malta who were vaccinated with only 2 doses equivalent by
15th December 2021, and not vaccinated with an additional dose by 8th
March 2022, and alive by 8th March 2022. Data were then matched with the
national testing database (all RT-PCR/Rapid Diagnostic Tests - RDT
tests) until 8th March. We collected vaccination status, vaccine brand,
vaccination date, infection history, and age. Using logistic regression,
we examined different combinations of vaccine dose, prior infection
status and time, and the odds of infection during the Omicron period
(December 15th, 2021 onwards). Results: Results found that participants
infected with Sars-Cov-2 prior to the Omicron wave had a significantly
lower odds of being infected with the Omicron variant. Additionally, the
more recent the infection and the more recent the vaccination, the lower
the odds of infection. Receiving a third dose within 20 weeks of the
start of the Omicron wave in Malta offered similar odds of infection as
receiving a second dose within the same period. Conclusion: Time since
vaccination is a strong determinant factor against infection, as was
previous infection status and the number of doses taken. This finding
reinforces the importance of future booster dose provision especially to
vulnerable populations.