Article Notes
The usual 'correlation is not causation' qualifier is very necessary here. Perhaps doubly-so because this is a meta-analysis using pooled data of over 1 million people.
It's completely possible that someone who chooses to receive an influenza vaccine is also someone who makes more cautious health decisions and is more risk-averse, reducing their COVID exposure risk. The vaccinated are also more likely to be from better health-serviced locations and probably from higher socio-economic groups.
Still, interesting association nonetheless.
This study is primarily an evaluation of a specific RAT, the Coris coronavirus disease 2019 Ag Respi-Strip test – which while showing poor sensitivity, it again demonstrates that RAT-positivity correlates with viral load. This study used PCR-detection as the gold standard for evaluation, which because of the extreme sensitivity of PCR testing, does not easily translate to assessment of contagiousness (although Ct > 25-30 seems to correlate with inability to grow in viral culture).
Early 2021 update of the 2020 Cochrane Review: Rapid, point-of-care antigen and molecular-based tests for diagnosis of SARS-CoV-2 infection.
Better data is now available on correlation between PCR cycle threshold and likely infectivity.