Preliminary results from a Spanish study on mixing COVID-19 vaccines have revealed that administering a dose of Pfizer vaccine to those who have already been given a first dose of AstraZeneca vaccine is highly safe and effective. 

The Combivacs study noted that the presence of IgG antibodies was around 30 to 40 fold higher in the individuals who received a second dose with the Pfizer vaccine compared to a control group that received only one AstraZeneca dose. The presence of neutralizing antibodies was found to increase seven times following a dose of the Pfizer vaccine, which was significantly more than the doubling effect noted following a second dose of the AstraZeneca vaccine… (ET Healthworld – Reuters, May 19, 2021)