You can look at it like that, but they are actually hoping for better, as the vaccine has an effect that even if you still catch covid-19 after vaccination, it would appear that you will, probably, be less seriously ill.
The problem for the authorities with that argument is that, whilst it is (probably) true that once vaccinated you will be less seriously ill if you do still catch it, there isn't much evidence that it stops you catching it. If the vaccine turns people who would otherwise have gone down with a mild dose of man-flu into asymptomatic cases, then those people can catch it and may not know they're infected. Since they're vaccinated they may think they're immune. That, together with these new much more easily caught mutations, could turn vaxinated under 50's into super-spreaders.
I'm also not sure where the 90% comes from wrt the Oxford Vax - the version we're most likely to get. The figures published in the Lancet (
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32661-1/fulltext) for two 100% doses up to 6 weeks apart was 60-70%. The 90% figure refers to the 50%-100% Oxford doses which haven't been approved by the MHRA because the sample size was too small, and all the participants were in the 40-55 age group IIRC. We already know that those aged 60+ are at a much higher risk, so if the 50-100 regime had been tested across a wider age group the results would probably be lower (a lot lower) than 90%.
You can go further and say that if the 100-100 dosage has an average of 60-70% across all tested age groups, but the 40-55 age group has a 90% rate for a 50-100 dose, then it's highly likely the 40-55 group has a similar 90% ish rate for a 100-100 dose. If that's the case, then because 60-70% is the average for all age groups, the 55-90 year old rate must be waaaaayyyyyy less than 60-70%.
In short, the vaccines are good news, and should help to dramatically reduce serious cases and hospital admissions once the majority of over 60's have been jabbed. But the over 60's are going to remain much more vulnerable than the under 40's for a long time yet. IMV.
It's also interesting that the Oxford testing was done in the UK, South Africa and Brazil. Remind me where these new strains of the virus are bing first detected
