CNN

Germany has issued what appears to be the strongest recommendation anywhere for the mixing of Covid-19 vaccines on efficacy grounds.

The German Standing Committee on Vaccination (STIKO) said Thursday that people who receive a first dose of the Oxford-AstraZeneca vaccine "should get an mRNA vaccine as their second dose, regardless of their age."

This makes Germany one of the first countries to strongly recommend that people who have received a first dose of AstraZeneca receive either a Pfizer-BioNTech or Moderna vaccine as their second dose.

German Chancellor Angela Merkel helped pave the way for mixed vaccine use when she received the Moderna shot in June as her second dose following a first dose of the AstraZeneca vaccine.

STIKO said that "current study results" show that the immune response generated after a mixed dose vaccination "is clearly superior."

The mRNA vaccines currently approved by the European Medicines Agency (EMA) are Pfizer-BioNTech and Moderna.

Canada's National Advisory Committee on Immunization made a weaker recommendation on June 17 when they said that "an mRNA vaccine is now preferred as the second dose for individuals who have received a first dose of AstraZeneca/COVISHIELD vaccine."

'Better immune response'

The Canadian committee said it was making the recommendation based on "emerging evidence of a potentially better immune response from this mixed vaccine schedule."

A study carried out by researchers at the University of Oxford and published June 28 found that "alternating doses of the Oxford-AstraZeneca and Pfizer-BioNTech vaccines generate robust immune responses against COVID-19."

According to a University of Oxford press release, the paper found that "both 'mixed' schedules (Pfizer-BioNTech followed by Oxford-AstraZeneca, and Oxford-AstraZeneca followed by Pfizer-BioNTech) induced high concentrations of antibodies against the SARS-CoV-2 spike IgG protein when doses were administered four weeks apart."

The EMA said in a press briefing on Thursday that although they are not "not in a position to make any definitive recommendation on the use of different Covid-19 vaccines for the two doses" there is a "strong scientific rationale" behind the approach.

Marco Cavaleri, head of Biological Health Threats and Vaccines Strategy for the EMA, told the briefing that the agency is "aware of the preliminary results from studies conducted in Spain and Germany" that "show that this strategy achieves satisfactory immune response and no safety concerns."

Also making reference to the recent Oxford data, Cavaleri said the EMA would continue to review the data as it becomes available.

Cavaleri affirmed that although the EMA makes recommendations "based on all the available evidence on the benefits and risk of a specific vaccine," the responsibility for how the vaccination should be administered falls to "the expert bodies guiding the vaccination campaigns in each member state."

Some European countries have previously administered mRNA vaccines as the second dose following a first dose of AstraZeneca on health and safety grounds, rather than for efficacy.

Following concerns about potentially fatal blood clotting incidents, countries such as Germany and Spain recommended that people under the age of 60 who received a first dose of AstraZeneca should a receive a mRNA dose for their second dose.

In making their recommendation on May 21, the Spanish Bioethics Committee said that although they recommended people who had a first dose of the AstraZeneca vaccine to receive a second dose of an mRNA vaccine, they would prefer people taking a second dose of AstraZeneca over no second dose at all.

New wave fears

Germany's updated guidance comes as the World Health Organization (WHO) warned that Europe was risking a new wave in August due to the relaxation of restrictions, the spread of an infectious Covid-19 variant and low vaccination coverage.

"Last week, the number of cases rose by 10%, driven by increased mixing, travel, gatherings and easing of social restrictions," Hans Kluge, WHO Regional Director for Europe, said Thursday in a statement, as he warned that the Delta variant would be dominant in the region by the end of the summer.

Some 63% of Europeans are waiting for their first jab, he said. yet Europe "will still be mostly restriction-free, with increasing travels and gatherings" in August.

"The three conditions for a new wave of excess hospitalizations and deaths before the autumn are therefore in place: new variants, deficit in vaccine uptake, increased social mixing," he said. "There will be a new wave in the WHO European region unless we remain disciplined, and even more so when there are far fewer rules in place to follow -- and unless we all take the vaccine without hesitation when it is our turn."

Kluge stressed two doses of the vaccine were effective against the Delta variant. "But the truth is that the average vaccine coverage in the region is 24% only, and more serious, half of our elders and 40% of our health care workers are still unprotected," he said.

"With these figures, nowhere is the pandemic over, and it would be very wrong for anyone -- citizens or policy makers -- to assume that it is," he said.