Some people waiting to get their second dose of the coronavirus vaccine may be wondering if it's OK to mix Pfizer and Moderna shots.
A trial is being launched in the United Kingdom to explore just that. The Centers for Disease Control and Prevention recently took a stance, and health experts are sharing their opinions, too.
"What we've seen in mixing vaccines in the past suggests it should be safe," said David Pride, M.D., P.h.D., an infectious disease specialist at UC San Diego Health.
Part of Dr. Pride’s analysis is based on the fact that the Pfizer and Moderna vaccines are similar because both use Messenger RNA (mRNA) technology.
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But the infectious disease specialist is quick to note tests are underway to make sure mixing the vaccines don't cause any harmful effects while still providing the same protection.
“The question becomes if you receive one vaccine, both of which target spike protein on the virus, and then a different vaccine, will you still stimulate that memory response or will your body remember and respond appropriately the second time? We believe these vaccines are similar enough that the answers are yes, but it hasn't been studied yet," said Dr. Pride.
Mixing doses is acceptable under “exceptional situations,” according to the CDC -- "exceptional" included when it can’t be determined which vaccine a person took first, or if that dose no longer available after 28 days.
However, the CDC acknowledged it hasn’t tested to see if their recommendations would change the safety or effectiveness of either vaccine and said the vaccines aren’t interchangeable under normal circumstances.
The CDC also notes that it’s OK to wait up to six weeks to get the second shot of either company's vaccine.
The possibility of having to mix vaccines became closer to reality as severe weather in other parts of the country caused persistent vaccine delivery disruptions and highlighted the vulnerability of the coronavirus vaccine supply chain.
San Diego County appears to be bouncing back from the disruption, which forced the temporary closure of vaccination sites and thousands of appointment cancelations.
As for the history of mixing vaccines, Dr. Pride said there have been times when it's showed promise.
“Previous vaccinations, you've been able to mix them. And in some cases they could lead to better response in others, the same. In this case we don't know for certain" he said.
Only testing and time will tell.
That lack of data is also why the World Health Organization also says the vaccines for now aren’t interchangeable.