(CN) — Pharmaceutical companies are holding out hope that the mRNA technology used to develop breakthrough Covid-19 vaccines is flexible enough to provide for seasonal shots in case immunity gained from initial vaccination is short-lived.
The first Covid-19 vaccine approved for use in the U.S., Pfizer and BioNTech's shot was purportedly 95% effective in preventing symptomatic coronavirus infections in a large study group. Positive study results have also been released for Moderna's vaccine, which secured an emergency use authorization by the Food and Drug Administration on Dec. 18.
The jury is still out on how long the immunity induced by these rapidly developed mRNA vaccines will last.
MRNA vaccines do not trigger immune responses with a weakened or inactive virus as do many traditional viral vaccines. Instead, they send molecular instruction codes – the mRNA – into cells, prompting the cells to produce a protein common to the novel coronavirus. The immune system recognizes the protein as foreign and develops a defense that later goes into action upon encountering the real virus.
Despite the encouraging clinical trial results, Pfizer is being cautious about speculating on the durability of immune response elicited by mRNA vaccines.
"We don’t know how the virus will change, and we also don’t know how durable the protective effect of any vaccination will be," the company said in a statement.
Pfizer said that if it turns out that the induced immunity lasts only a few months, mRNA vaccines are suitable for repeated administration as booster shots.
If a mutation in the Covid-19 virus affects Pfizer's vaccine effectiveness, the company said, mRNA technology will enable "rapid development" of adjustments. The technology also allows for a fast production process without the need for complex mammalian cell systems used in traditional vaccine production, the company said.
Hundreds of thousands of people in the U.S. have received Pfizer's vaccine since the FDA’s Dec. 11 decision to approve its use on an emergency basis. Administration of Moderna's vaccine to the public began this week.
Multiple other Covid-19 vaccine candidates using a variety of technologies are under development by other large pharmaceutical companies. Viral-vector vaccines, one from Johnson & Johnson, and another from a partnership between AstraZeneca and Oxford University, are in late-stage clinical trials.
Dr. Marc Hellerstein, a biochemistry lab head at the University of California, Berkeley, said researchers will be focused next year on determining which vaccine produces the longest-lived immune response against Covid-19. Of particular importance is the immune response of T cells including CD8+ cells, immune system agents that kill off virus-infected cells in the body, Hellerstein explained.
Studies have found that people infected with SARS-CoV – a similar coronavirus that caused a 2003 outbreak in Asia – had natural T-cell activity that lasted for several years. Levels of antibodies against that virus appear to have been relatively short-lived, however. One study showed that three years after the 2003 outbreak, antibodies were detected in only half of patients. Antibodies work in the extracellular space, finding and neutralizing free-floating viral particles.
"We know that people who recovered from serious betacoronavirus infection, prior to Covid-19, had long-lived T cells and lousy antibody durability," Hellerstein said of patients' natural immune response.
Several pharma companies have touted seemingly robust T-cell responses induced by their Covid-19 vaccines. For one, Pfizer's FDA briefing stated that nearly 90% of study participants who received Pfizer's vaccine showed CD8+ T-cell production against the targeted coronavirus protein.
"An important feature of mRNA vaccines is that they stimulate effective T-cell and T-cell memory responses. This ensures longer term protection from viral infection and disease," Kathrin Jansen, Pfizer's head of vaccine research, told an FDA advisory panel two weeks ago.