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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.

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.

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.