• Health experts generally agree that the mixing and matching of the vaccines should be safe.
• But clinical trials are ongoing.
• The Centers for Disease Control and Prevention maintains that the safety and efficacy of a mixed-product series have not been evaluated.

With new guidance following reports of rare blood clots, the global medical community is considering whether it's possible, and safe, to administer two different vaccine candidates to the same person.    

This week the European Medicines Agency and the U.K.'s Medicines and Healthcare products Regulatory Agency both determined that there is a possible link between the AstraZeneca-University of Oxford vaccine and very rare cases of unusual blood clots with low blood platelets.

Neither the European nor the U.K. health regulators recommended age restrictions in using the vaccine. However, the U.K. regulator did note that the data suggested that there is a slightly higher incidence reported in the younger adult age groups and therefore advises that this evolving evidence should be taken into account when considering the use of the vaccine. 

The EMA similarly reiterated that the vaccine is safe and effective, but noted that use of the vaccine at a national level will also take into account the pandemic situation and vaccine availability in the individual countries.

Subsequently the U.K., various EU countries and other governments around the world have moved to recommend using alternative vaccines for younger people. 

Given the change in guidance, younger people are now asking the question: If I've already received one dose of the vaccine, should I come back for the second one?

Governments have different answers to this question. Health experts generally agree that the mixing and matching of the vaccines should be safe. But clinical trials are ongoing.

Guidance varies

The U.K.'s Joint Committee on Vaccination and Immunisation advises: "All those who have received a first dose of the AstraZeneca COVID-19 vaccine should continue to be offered a second dose of AstraZeneca COVID-19 vaccine, irrespective of age. The second dose will be important for longer lasting protection against COVID-19."

In contrast, France's health regulator recommends that people under the age of 55 who have received their first dose of AstraZeneca should get Pfizer or Moderna for their second shot. It advises a gap of 12 weeks between these first and second shots in these instances. The regulator stated there is no reason to fear specific adverse events if you had the first AstraZeneca jab and then switch to an mRNA jab for the second.

Germany has followed a similar path. The German vaccine committee recommended that people under the age of 60 who had received one shot of the AstraZeneca jab should opt for a different vaccine for their second dose.

Trials underway

"The guidelines are the guidelines. But as a basic immunologist, can I see any argument why it would be unsafe or poor practice to mix and match vaccines? No, I can't see any at all. It would still induce great immunity. No problem with that whatsoever," Danny Altmann, a professor of immunology at Imperial College London told CNBC's "Squawk Box Europe" Friday.

Andrew Freedman, a reader in infectious diseases at Cardiff University's School of Medicine told CNBC: "Studies are ongoing to look at the concept of mix and match. There's no theoretical reason why that shouldn't be feasible and safe but we have to wait for these studies."

In terms of a possible booster dose that may be needed in the fall or winter, he added: "I don't think there's any real concern that you wouldn't be able to follow two doses of the AstraZeneca vaccine with one of the other messenger-RNA vaccines."  

Meanwhile, Franz-Werner Haas, the CEO of vaccine maker CureVac told CNBC this week: "The good news is all these vaccines are coding for the same spike protein ... so therefore there are clinical trials and data seen that you can mix and match these different vaccine platforms." 

"So in this regard, I have very high hopes that this is going to work out quite well," he added. 

CureVac's own candidate is still in clinical trials. Data readout is on track for the second quarter of this year.

The Centers for Disease Control and Prevention maintains that the safety and efficacy of a mixed-product series have not been evaluated.

Several trials are underway to look at the effects of mixing vaccines. The U.K. launched a trial in February looking specifically at mixing the AstraZeneca-Oxford vaccine with the Pfizer-BioNtech shot. Findings are not expected to be available until the summer. Separately, trials are being held looking at a combination of the AstraZeneca-Oxford and the Russian Sputnik V vaccines.