Scotland Study on Vaccine Effectiveness (VE) of 2 doses AZ & Pfizer doses against all COVID and Symptomatic Infections 
Pfizer vaccine provides excellent protection against Delta variant
Less protection with AZ
Is it time to consider boosting AZ with Pfizer as second dose?
Heterologous boosting is safe  Interim safety analysis in the UK mix-and match study, showed an increase systemic Adverse Effects Following Immunization (AEFI) in the AZ-Pfizer heterologous vaccine schedule compared to the AZ-AZ , Pfizer-Pfizer homologous group.
This was associated with increased paracetamol use.
It is reassuring that the systemic AEFI were short-lived.
Pfizer booster triggered a stronger and more robust antibody response than 2 doses of AZ. [3,4] The neutralizing antibody increased 7X after a booster Pfizer dose, doubled that with a second AZ dose in the Spanish study
The IgG antibodies were 30-40X higher than in a control group who received only 1 AZ dose.
Only 1.7% reported systemic AEFI namely headache, myalgia and malaise.
Malaysia has a window of opportunity to boost the millions of first dose AZ vacinees with a second Pfizer dose and achieve a better immune response and hence protection namely against the Delta virus.
Many Western nations are immunizing their younger vacinees (less than 50 years old) on AZ first dose with a second Pfizer dose due to the relatively increased risk of blood clots (TTS).
The JKJAV and MOH need to consider these various vaccination strategies, including single dose mRNA vaccines in previous COVID patients, prolonging second dose (with careful genomic surveillance) within the context of restricted and delayed vaccines supply chains.
These break-through live saving vaccine strategies can only be realized and operationalized if the experts are thinking ahead and the MySJ app is robust and contemporary.
It is now even more pressing with the transmissibility of the new variant which is challenging the vaccine rollout programs of the better immunized western nations e.g. the UK, the US and Canada