Nictrix said:
Can anybody explain the change from it supposed to be 3 weeks between the vaccines to now being 12 weeks in between them when the professionals seem to say it should be a maximum of 42 days?
When the vaccine trials were conducted, the manufacturers followed a specific protocol, where there was an interval of, say, 3 weeks between the first and second dose of the vaccine under test, therefore, all the data recorded and presented to the various regulation authorities relates only to such conditions and licensing is granted on that basis. Obviously, not all vaccinated individuals were given their second dose on the stroke of midnight of day 21, and there would be some variability in the actual intervals between the administration of the 2 doses, and the data will probably contain information for some individuals who received their second dose after a much greater interval than 3 weeks. I understand the Oxford/AstraZeneca trial included some variable intervals between administration of the 2 doses.
Under the principles of vaccination, the administration of a single dose of a vaccine will initiate an immune response which will develop in the few weeks that follow, which will provide a period of immunity to the vaccinated individual which will last for a few months at least. The administration of a second dose of the vaccine will "boost" the initial level of immunity, which should prolong the immunity for a much longer period, providing protection to the individual.
The logic is now of providing a first vaccine dose to as many people as possible in order to give initial immunity to a much higher percentage of the most vulnerable groups within a 12 week period, rather than providing a much smaller group a longer lasting level of immunity, before the second dose is administered to this larger group.
Now that the vaccination programme is well underway, the researchers/statisticians have access to a considerably greater amount of data in "real world" use than the manufacturers had during their trials, therefore it's now possible to assess antibody levels/immunity amongst all cohorts of vaccinated people (the most vulnerable) and not just healthy volunteers, as in the trials. It would appear that the scientists advising the government are acting empirically from more information gathered from a very large sample group (the vaccinated population), rather than restrictively from information gathered from a much smaller sample group during clinical trials to "suck and see" if their vaccines were effective or not.