The Pfizer and Moderna COVID-19 vaccines offer some immunity after the first dose, but the clinical trials suggest you need that second dose to achieve maximum protection.

Studies have shown that the Pfizer vaccine offers at least 52% immunity to COVID-19 after the first shot and achieves full protection after a week following the second dose, which is administered 21 days later.

The Moderna drug recommends that doses are spaced 28 says apart. According to Newsweek, clinical trials have shown that, for the Moderna vaccine, the 94.5% efficacy reaches its peak 14 days after the second dose and the first dose offers up to 80% protection.

This information comes from a small trial, according to the Food and Drug Administration (FDA), and the agency warns that the sample size and the duration of study “limits the interpretation of these results.”

“There appears to be some protection against COVID-19 disease following one dose; however, these data do not provide sufficient information about longer term protection beyond 28 days after a single dose,” said the FDA.

Researchers have differed on how efficient one dose of the vaccines can be against COVID-19. For example, Newsweek reports that that the U.K.’s Joint Committee on Vaccination and Immunization found that the Pfizer drug has up to an 89% efficacy between days 15 and 21 after the first dose. According to SF Gate, other doctors and scientists also believe that a single dose of the Pfizer vaccine has an 80-90% efficacy. The second dose provides 95% immunity to people who are then almost totally protected against serious illness from COVID-19.

However, even though 80% to 90% efficacy sounds impressive, top officials at the FDA caution that people should adhere to the recommended two doses to achieve full protection and not make any changes to the dosing schedule. FDA Commissioner Dr. Stephen Hahn and Dr. Peter Marks, of the FDA’s vaccine division, said in a statement:

“Without appropriate data supporting such changes in the vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19.”

It’s is important to remember that building immunity takes time, says Very Well Health. For the Pfizer vaccine, data shows it takes at least a week after the two doses before your body builds a strong defense against COVID-19.  The Moderna drug takes at least two weeks. Experts say you should still wear a mask and physically distance even after getting your shots. And while scientists hope that immunity will last for years, the actual duration is unknown.