I get that, though as you mention this is for the oxford one, yet they are also doing it with the pfizer one despite no direct evidence.
FTFY
There's no direct evidence, but there is indirect evidence. The human immune system still works the same way, regardless of what the bottle says.
IIRC (my virology was 20 years ago, and cursory), typically when vaccines require 2nd doses, the first gives you a decent response, and immunity; but the second gives you both a top-up, and prolonged immunity. As far as these vaccines are concerned, we simply don't know - we cannot know anything "long-term" about a virus that was first identified about a year ago.
Personally, I'm intinctively uncomfortable with a 12 week wait between doses of the AstraZeneca vaccine - but that's based on gut feel, with no evidence - AstraZeneca and Oxford University themselves seem to be comfortable with 12 weeks, then they know a **** load more than anyone here.
For the Pfizer vaccine, they haven't tested anyone at all with a 12 week wait; so that doesn't sound worth the risk - but it's still an educated guess based on how vaccines work, not just a panic measure pulled out of Johnson's johnson.
As ever with this unpresidented pandemic, I'm not going to overly castigate anyone for making mistakes, or taking a risk that doesn't pay off.
I reserve that for entirely foreseeable mistakes (such as directly undermining their own message and eroding confidence in the advice), going explicitly against the advice of those who know (which may include Pfizer, but I'd need to see what they say), and repeating previous mistakes (delaying inevitable action).
For AstraZeneca this is a calculated risk, based on (low-confidence) evidence. For Pfizer this is a calculated risk based on immune responces to vaccines.
The upside is that we get immunity into twice as many of our most vulnerable people in a short period of time.
Potential downside is that those same people may need a 3rd dose (once we've secured enough, and rolled out vaccination programmes nationally, and can be done with much less hassle).
Other risks are from the repeated mistake of muddled messaging and eroding confidence / support of recommended measures.
Bear in mind, a single dose of the AstraZeneca vaccine is still 53% effective at preventing symptomatic infection and 100% (low confidence) effective at preventing hospitalisation, whilst apparently also reducing spread.
www.theguardian.com
