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DiapDealer - the CDC guidelines regarding those who had J&J as their primary course needing mRNA boosters are similar to Australia's equivalent guidelines regarding boosters for Astra-Zeneca primary course recipients.
I had two shots of AZ as my primary course but 'had to have' Pfizer or Moderna for my third shot and and I assume my fourth shot, due late next month, will be the same.
The AZ and J&J products are viral vector vaccines - maybe it's a factor.
Novavax has been approved here, I wouldn't mind having a shot of that
BR