The effectiveness of Pfizer’s COVID-19 shot can drop to 83% within four to six months after getting the second dose of its vaccine. This is the latest indication that vaccine-induced immunity to the virus can wane and some kind of boost may be necessary in the future.

Pfizer Inc.

CEO Albert Bourla told investors on Wednesday that the shot provides 96% protection for the first two months, at least 90% effectiveness between the second and fourth months, and between 83% and 84% of protection for the fourth, fifth, and six months. 

“We will need a booster eight to 12 months from the second dose,” Bourla said, according to a FactSet transcript of the second-quarter earnings call. 

The drug maker has been making the case for booster shots, citing limited data from its own clinical research and real-world data out of Israel, where Pfizer’s vaccine is the predominant shot in circulation. 

“We do see—after six to eight months—more rapid waning concerning infections and mild to moderate symptoms,” Dr. Mikhail Dolsten, Pfizer’s chief scientific officer, said during the call. “Those are likely entirely, or to a large degree, dependent on antibodies and the drop in titer that we alluded to. If you raise it, you may have a good probability to reverse that waning.”

Still, there’s no simple black-and-white answer to whether booster shots are needed at this time. 

One, there is no definitive data. Pfizer said a preprint study detailing immunity levels will be published within the week, and it plans to submit clinical data for a third dose to the Food and Drug Administration in early August.

When speaking to investors this week, company officials attributed waning immunity levels in Israel to the much earlier vaccination campaign that rolled out in that country. (In fact, Israel announced Thursday that a third dose will now be available to people who are at least 60.) Pfizer also mentioned that there will be data in a few months coming out of the U.K., where doses were sometimes spaced out, that indicates “long-lasting protection.”

In addition, we still don’t know which people will need to boost their protection levels. Will the entire vaccinated population need a boost? What if third shots are only necessary for the elderly or people who are immunocompromised? What diagnostic test can be used to assess titer levels before giving a booster? How soon does this all need to happen? 

“While I cannot predict with certainty the future, I would not be surprised if, similar to flu, that we would need…to boost our vaccine against COVID,” Dolsten said. “Whether this will be on an annual [basis] or based on simple diagnostics that allow it to be boosted at the right time before your risk for infection is high, we need to monitor.” 

Either way, it’s a boon for Pfizer, as the legacy drug maker shifts into its new role as a vaccine leader.

Pfizer is expected to generate $33.5 billion in COVID-19 vaccine revenue this year, and Wall Street analysts have already baked boosters into their financial models for Pfizer and BioNTech SE
which developed the vaccine with Pfizer. 

“The largest remaining uncertainty is whether this third boost is simply the third dose in a three-vaccine schedule to achieve lasting, broad protection or is the first of a periodic (annual) boosted vaccine,” SVB Leerink analysts told investors on Thursday. 

Moderna Inc.
which developed the other FDA-authorized mRNA-based COVID-19 vaccine, is also testing booster shots in clinical trials. It has been much quieter in its communication around a third shot. The company is scheduled to share second-quarter earnings on Aug. 5.

Pfizer’s stock is up 16.3% so far this year, while the broader S&P 500

has gained 17.1%.

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