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Given the following post, respond as if you're commenting on Reddit:
human
Differences between Pfizer and AstraZeneca vaccine?

Some countries are getting Pfizer's vaccine some are getting the AstraZeneca. What are the major difference between the two? Are there benefits from one over the other? Is there a specific reason why a country like South Korea would opt for Astra's version?
gpt
Pfizer and Moderna are based on a new type of vaccine called mRNA.  These have never been approved for use in humans before outside of trials.  They must be kept at very cold temperatures (-70 and -20 Celsius respectively).  The testing of these vaccines has been for roughly 40k people for each company.  So far they’ve released safety and efficacy results based on 7 days after the administration of the second dose.  The phase 3 trial data has yet to be released in any peer review publications.  So far what we know is only from press releases from the manufacturers.  The reported side effects are said to be mild compared to the AstraZeneca and Sputnik V vaccines.  Medium to long term efficacy and safety of these vaccines are entirely unknown.  The uk has approved ‘emergency authorization use’ of the Pfizer vaccine (this is different than normal vaccine approval).  They have also granted Pfizer legal indemnity to protect them from lawsuits.

AstraZeneca and Sputnik V use a previously well known and in-use type of vaccine (called viral-vector, eg. some Hep B vaccinations).  They can be stored at normal refrigerator temperatures.  AstraZeneca has been tested on roughly 40k people, while Sputnik V has been tested on 20k people.  The side effects are said to be often more uncomfortable than the Pfizer and Moderna vaccines - typically headaches and/or soreness at the injection site. They have also only reported phase 3 results from only 7 days after the second dose.  They have also yet to release their phase 3 trial data in peer reviewed publications, having only released summaries so far in the form of press releases.  The mid and long term safety and efficacy rates of these vaccines is also entirely unknown.  The Russian Sputnik V vaccine is approved for use in Russia (possibly a few other countries as well).  It is the first vaccine to have been approved for public use, the Russian government having done so based only on phase 1 and 2 testing results.

All of the vaccines mentioned claim to have 90+% efficacy ratings, which is more than stellar for vaccines in general.  However it is widely expected that this efficacy rating will drop once much larger sample groups are known.  

I hope this helps.