A coronavirus vaccine being worked on by American researchers induces a robust immune response in healthy adults, results from early phases of clinical trials suggest.

Scientists found that the RNA vaccine, called BNT162b1, was generally well-tolerated, although some participants experienced mild to moderate side-effects.

These increased with dose level, in the seven days following vaccination, including pain at the injection site, fatigue, headache, fever and sleep disturbance.

According to an interim report of a phase 1/2 clinical trial published in Nature, the vaccine, which is being developed by Pfizer, elicited a robust immune response in participants, which increased with dose level and with a second dose.

The clinical findings for the BNT162b1 RNA-based vaccine candidate are encouraging and strongly support accelerated clinical development

Antibodies against Covid-19 were present 21 days after a single vaccination at all dose levels, and there was a substantial increase in Sars-CoV-2-neutralising antibodies seven days after the second dose was given.

The researchers wrote: “The clinical findings for the BNT162b1 RNA-based vaccine candidate are encouraging and strongly support accelerated clinical development, including efficacy testing, and at-risk manufacturing to maximise the opportunity for the rapid production of a Sars-CoV-2 vaccine to prevent Covid-19.”

Corresponding author Judith Absalon, senior medical director at Pfizer Vaccines Clinical Research, and colleagues reported interim data from an ongoing phase 1/2 clinical study of BNT162b1.

Forty-five healthy adults – 23 men and 22 non-pregnant women – aged 18 to 55 were randomised to receive 10 micrograms, 30 micrograms or 100 micrograms of BNT162b1, or a placebo.

Researchers found the immune response was much stronger in the 30 microgram group than in the 10.

However, there were no notable differences in immune response between the 30 and 100 groups after one dose, and, as participants who received the 100 microgram dose also experienced greater side-effects, they did not receive a second dose.

According to the study, levels of neutralising antibodies in participants were 1.9 to 4.6 times higher than those in patients recovering from Sars-CoV-2 infection.

But the scientists warned that, although this comparison provides a benchmark for evaluating the vaccine-elicited immune response and the vaccine’s potential to provide protection, phase 3 trials are needed to determine the efficacy of BNT162b1.

The publication of peer-reviewed data from our mRNA-based vaccine development programme against Sars-CoV-2 in a world-renowned publication like Nature provides further validation of our rapid progress toward developing a safe and effective potential vaccine to help address this current pandemic

The study is also enrolling adults aged 65–85, and later phases will prioritise the enrolment of more-diverse populations.

Delivered intramuscularly, BNT162b1, which encodes a Sars-CoV-2 receptor-binding antigen, is one of several RNA vaccine candidates that are being studied in parallel for selection to advance to a trial of their safety and efficacy.

RNA vaccine platforms, which use messenger RNA to elicit an immune response, are generally considered safe and have facilitated the rapid development of vaccines against Sars-CoV-2.

RNA is synthetic strands of genetic code, based on the virus’s genetic material.

Once injected into muscle, the RNA self-amplifies – generating copies of itself.

Kathrin Jansen, senior vice president and head of vaccine research and development, Pfizer, said: “The publication of peer-reviewed data from our mRNA-based vaccine development programme against Sars-CoV-2 in a world-renowned publication like Nature provides further validation of our rapid progress toward developing a safe and effective potential vaccine to help address this current pandemic.

“We are encouraged by the overall advancement of the programme and look forward to generating additional data from our ongoing studies.”

– Pfizer and BioNTech recently selected BNT162b2 as the vaccine candidate to progress to a Phase 2/3 study, which will enrol up to 30,000 participants aged between 18 and 85.