During the third week of July, the World Health Organization reported a 131% increase in new COVID cases in the US. With the delta variant of the coronavirus becoming the dominant strain, pharmaceutical giant Pfizer said it’s working on a booster shot for its COVID-19 vaccine. To prepare for the possibility, the US Centers for Disease Control and Prevention said it’s weighing a third shot for immune-compromised people.

In a July 8 press release, Pfizer said a third shot of its vaccine would enhance the immunity of those who’ve already received the first two vaccine shots. In addition to creating a booster for its existing vaccine, the drugmaker said it would formulate a new version of its COVID-19 vaccine to target the delta variant. The CDC and the Food and Drug Administration responded with a joint statement the same day saying that fully vaccinated Americans «do not need a booster shot at this time.»

But that was all before the CDC’s most recent internal document confirmed in late July that vaccinated people can both contract the highly contagious delta variant and easily spread it. Those recent findings were behind the CDC’s shift in its mask guidance, which now recommends indoor mask use for everyone in areas with high transmission rates. The debate over mask use and vaccine boosters underscores how scientists and other health experts continue to grapple with the uncertainties of COVID-19.

Here’s what we know about Pfizer’s plans for a booster, and why the CDC and FDA caution against it, at least for now.

What are Pfizer’s plans for a COVID-19 booster shot?

Along with Moderna, Pfizer’s current two-dose vaccine provides effective protection against all known variants of COVID-19, including the delta variant. Studies have shown the Pfizer vaccine to be over 90% effective against the virus. But Pfizer also said its own research showed a third booster of its current vaccine increased antibody levels five to 10 times higher over its two-dose shots. The company noted its results have not been published or peer-reviewed.

Pfizer said it believes the level of protection the two doses of its vaccine provide can decrease over time, and a third booster dose may be needed «within six to 12 months» after a person is fully vaccinated. To prepare for the booster, Pfizer is testing both the effectiveness of a third dose of its current vaccine and working on an updated version targeting the delta variant. Pfizer said it would begin clinical trials on the booster this month as it seeks approval from government regulators for a third dose.

The company says a third shot given at least six months after the second shot in its original vaccine series would enhance protection against the delta variant, which has been known to infect fully vaccinated people.

What are the CDC and FDA saying in response?

«People who are fully vaccinated are protected from severe disease and death, including from the variants currently circulating in the country such as Delta,» the CDC and FDA said in a July 8 joint statement, without naming Pfizer. The government agencies emphasized the need for all eligible people to receive full doses of one of the approved vaccines, all of which are free.

The CDC and FDA said the question of a booster requires extensive scientific data and doesn’t depend on the input from pharmaceutical companies alone. «Virtually all COVID-19 hospitalizations and deaths are among those who are unvaccinated,» the statement mentioned, adding that the agencies will approve booster doses «if and when the science demonstrates that they are needed.»

According to a July 23 article in The New York Times, there is a growing consensus among Biden administration health officials that older individuals and those with compromised immune systems may, in fact, need a third shot. The same article notes that the CDC is exploring options to administer third doses even prior to authorization.

Is Moderna also planning to develop a booster shot?

While scientists and public health officials continue to study if those who are fully vaccinated will need a booster shot, Moderna said — along with Pfizer — it is moving ahead and exploring the need for a third shot.

Would the booster shot be free?

The current one-dose vaccine shot from Johnson & Johnson and two-dose versions from Moderna and Pfizer are free to anyone who wants to get vaccinated. According to the Biden administration, COVID-19 booster shots will also be free, if and when they’re approved.

Is it a good idea to mix and match COVID vaccines?

The CDC doesn’t recommend mixing and matching vaccines from the different makers, saying it hasn’t evaluated the effectiveness of mixing vaccine doses and that the «vaccines are not interchangeable.»

However, other global health agencies and countries are testing administered vaccines from two different manufacturers. In England, for example, a recent study found that those who received a first dose of the AstraZeneca vaccine and a second of Pfizer had a higher immune response than those who received two doses of the AstraZeneca vaccine.

While we wait to see how the situation develops, here’s what we know about the delta variant, more about COVID-19 boosters and if you need to continue to wear a mask.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.