A new pre-print study published this month by researchers (Hunter et. al.) from King College London and University of East Anglia based on data from Israel indicates that the Pfizer vaccine appears to confer high effectiveness 21 days after the first dose.
The caveat is that the vaccine effectiveness was essentially zero up until Day 14 post injection, and then rises steadily until Day 21 post injection to around 90%.
The second dose would raise the vaccine effectiveness to the much publicized 95%, and more importantly, confer a long term protective effect.
What this means is that for those that have received the firs Pfizer dose must not lower their guard when interacting with others; as the data shows, the first shot essentially provides no protection until 2 full weeks has transpired. This is likely due to the fact that the vaccine is mRNA based and the body needs time to produced the viral protein coded by the mRNA, and then produce antibodies against the viral protein.
In other words, just because you went and received a vaccine shot, doesn't mean you can drive over to your grandma's house the next day and hug and kiss her.
It should also be noted that even if you have received both shots, it doesn't mean you can head over to the local bar and down a couple of pints with your buddies. 95% effectiveness is not 100% effectiveness, and scientists have cautioned that you may still catch and transmit the SARS-CoV-2 virus even after being vaccinated. This is because the primary effect of the vaccine is to strongly decrease your chances of suffering from severe illness.
Another caveat is that we still do not know how much the new strains from South Africa and Brazil has reduced the vaccine effectiveness. So far the AstraZeneca (adeno-virus vector based versus m-RNA based) vaccine seems to be ineffective against the South African strain when it comes to mild and moderate disease states; so much so that the South African government has halted the AstraZeneca vaccine rollout. The good news so far is that the Pfizer vaccine appears to still confer high effectiveness against the newer UK strain that is now spreading rapidly across the United States and several other nations.