Germany's vaccine committee on Thursday recommended that everyone who received an AstraZeneca first dose switch to BioNTech-Pfizer or Moderna jabs for better protection against the coronavirus, including against the Delta variant.
Studies show that the immune response is "clearly superior" when an AstraZeneca shot is combined with a second mRNA vaccine, compared with double AstraZeneca jabs, said STIKO.
The commission therefore recommended the mix "regardless of age" and with a minimum gap of four weeks between the two jabs.
The vaccines developed by BioNTech-Pfizer and Moderna are based on the same novel messenger RNA technology, which trains the body to reproduce spike proteins, similar to that found on the coronavirus.
When exposed to the real virus later, the body recognises the spike proteins and is able to fight them off.
Viral vector vaccines like AstraZeneca and Johnson & Johnson use genetically-engineered version of a common-cold causing adenovirus as a "vector" to shuttle genetic instructions into human cells.
Chancellor Angela Merkel had received a Moderna second jab after getting an AstraZeneca injection for her first.
Pointing to studies in Britain, STIKO said protection against the Delta variant is "significantly reduced" after only one shot.
"In this context, STIKO points out that it is important to get the second vaccine dose," it said.
Germany expects the Delta variant to take over as the dominant strain of the coronavirus in the country in the coming days.
"I expect that in the course of July we will see Delta accounting for over 70 to 80 percent of infections in our country," Health Minister Jens Spahn told reporters.
The Robert Koch Institute (RKI) health agency reported on Wednesday that the Delta variant, first identified in India, already accounted for 37 percent of infections in Germany the week to June 20.
The variant is expected to account for at least half of all new infections by this week, the experts said.
If the Delta variant becomes dominant in Germany, so-called coronavirus variant countries such as Britain and Portugal -- from which most travel is currently banned -- could be reclassified, Spahn said.
Given the increasing spread of Delta and research showing that full vaccination protects well against it, "we will look at the situation in the next few days", Spahn said.
"If both of these things are confirmed, we will then be able to treat Portugal and the United Kingdom as high-incidence areas", rather than variant countries, he said.
Only citizens and residents of Germany are permitted to enter from a variant country and are subject to a two-week quarantine, regardless of whether they are fully vaccinated or can provide a negative Covid-19 test.
By contrast, anyone can enter from a high-incidence country and can end their quarantine after five days with a negative test.
The current variant countries list includes Britain, Portugal, India and Russia.
Despite the rising share of the Delta variant, the overall incidence in Germany has been steadily declining in recent weeks.
The RKI on Thursday reported 892 new cases in the past 24 hours and an incidence rate of 5.1 new infections per 100,000 people over the last seven days.
But across Europe, the number of new cases rose again last week after 10 weeks of decline, according to the World Health Organization.
Speaking in the European Parliament, European Central Bank chief Christine Lagarde also warned of the threat posed by virus variants to the economy.
"The nascent recovery still faces uncertainty, also due to the spread of virus mutations," she said.