People who received two shots of the Moderna Covid-19 vaccine in its earliest, Phase 1 clinical trial are being offered a third shot, a so-called booster, as part of a continuing study to determine whether repeated vaccinations are needed and whether they are safe and effective, the company said on Thursday.
The vaccines made by Moderna and Pfizer, both using genetic material called mRNA, were shown in clinical trials to be about 95 percent effective at preventing symptomatic cases of Covid-19 when administered as a two-shot regimen. But they have not been in use long enough to tell how long the immunity lasts or whether additional booster shots will be needed.
“We anticipate that an additional dose could further boost the levels of neutralizing antibodies, should such a boost be required, and that this is expected to be an advantage of mRNA vaccines,” Colleen Hussey, a Moderna spokeswoman, said in a statement.
The immune system makes neutralizing antibodies in response to a virus or a vaccine, and can block the virus from breaking into cells.
The company’s statement said the boosters were being offered to participants six to 12 months after their second shot. Volunteers in early trials received different vaccine doses as part of the company’s attempts to calibrate the most effective amount of active ingredient to use.
The Moderna vaccine was ultimately given an emergency green light from the Food and Drug Administration as a two-dose vaccine of 100 micrograms of mRNA each. Trial volunteers who got lower doses might especially benefit from a third shot, said Akiko Iwasaki, an immunologist at Yale University.
In its statement, Moderna said that boosters might also be studied in people who took part in its later, Phase 3 study of 30,000 participants, “if accumulating antibody persistence data indicate that this is warranted.”
Company officials said at a conference this week that they thought protection should last at least a year, according to a report by CNBC. But Moderna’s chief executive, Stephane Bancel, also said in an interview with CNBC that the vaccine might have to be adjusted in the future to immunize people against new coronavirus variants or strains, much as flu vaccines are regularly revamped.