As they say:
In 971 individuals who received two doses of the Pfizer-BioNTech vaccine, the proportion testing positive was high across all age groups, at 91.1% (88.5, 97.1) overall. Following a single dose of Pfizer-BioNTech vaccine after 21 days or more, 84.1% (82.2, 85.9) of people under 60 years tested positive (unadjusted) with a decreasing trend with increasing age, but high responses to a single dose in those with confirmed or suspected prior COVID at 88.8% (85.9, 91.2) overall. (Table 4) The apparent higher positivity in people with prior COVID-19 was present in all age groups.
I suggest the evidence is abundant: two doses are needed, and the sooner the better.
A single dose does not appear to be achieving anything like the expected benefits, most especially for the vulnerable.
On this basis, government policy would appear to be placing wholly inappropriate faith in single-dose jabs that do not deliver vaccination for those who really need it.