Pfizer-BioNTech Covid-19 Vaccine Shows Phase I Promise

A vaccine made out of messenger RNA (mRNA), developed by the US pharmaceutical giant Pfizer and its German partner BioNTech, has produced antibodies against the Covid-19 pathogen in healthy volunteers.

The results come from an interim analysis of an ongoing phase I clinical trial in 45 participants. So far, all test subjects who received two injections of the vaccine candidate have developed antibodies against SARS-CoV-2, the virus behind Covid-19. The blood levels of these antibodies in vaccinated subjects also significantly exceeded those of patients that have recovered from Covid-19. 

With no serious side effects reported so far, a decision on whether to launch a phase II/III trial could come as early as this month.

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BioNTech and Pfizer are developing four different mRNA vaccine candidates against Covid-19 and the interim results come from the most advanced of the four. All of the candidates consist of delivering mRNA molecules into the body that cause the patient’s cells to produce fragments, or ‘antigens,’ of the Covid-19 coronavirus. The immune system then detects these antigens and produces antibodies ready for when the real virus shows up.

Two of the candidates carry mRNA instructions for making an antigen found on the surface of the novel coronavirus called the spike glycoprotein, which is thought to play a key role in the virus’ ability to enter cells. The other two candidates — which include the one that produced the latest clinical interim results — encode a small part of the spike glycoprotein called the receptor-binding domain. 

The receptor-binding domain candidates contain the piece of the spike that we think is most important for eliciting neutralizing antibodies for inactivation of the virus,” a Pfizer representative told me.

In addition, the four Covid-19 candidates developed by Pfizer and BioNTech use different mRNA formats that tweak key drug characteristics such as immunogenicity. The partners’ most advanced Covid-19 candidate comes in a format called nucleoside modified mRNA, which is designed to stop the immune system from attacking the drug molecules before they reach the cells.

Most traditional vaccines deliver antigens to stimulate the body’s immune system to develop corresponding antibodies. But because mRNA vaccines instruct the patients’ own cells to build the antigens, they could have a faster development than traditional vaccines.

mRNA vaccines have the potential to offer rapid manufacturing, including to large scale,”  Sarah Gilbert, Professor of Vaccinology at the University of Oxford, UK — who wasn’t involved in the study — told me. 

Up to now, there has been very little data on safety or immunogenicity of this new technology in clinical trials, but the pandemic has provided both the need and the opportunity to assess that. In a few months’ time, I expect the world will have a great deal more data on how this technology performs in comparison to more established technologies.

Of the dozens of vaccine candidates in development against Covid-19 globally, a large group of them uses mRNA technology. One of the most advanced mRNA candidates is being developed by the US biotech Moderna, which released positive interim phase I results in May. Moderna’s project is about to enter phase III trials, though international media recently reported a likely delay in those plans due to last-minute protocol changes.


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