This article was updated on 11/3/21.

Earlier this year, health care providers began administering the first doses of Johnson & Johnson’s COVID-19 vaccine, bringing the total number of vaccines available in the U.S. to three. This newest vaccine is a little different from the vaccines created by Pfizer and Moderna. Here’s what you need to know.

How is the Johnson & Johnson vaccine different from the other two available vaccines?

There are several differences between the three vaccines that are currently available in the United States. The first two vaccines approved for use — made by Pfizer and its partner BioNTech and by Moderna — require two shots, given several weeks apart, while the Johnson & Johnson vaccine only requires one shot. Additionally, both the Pfizer and Moderna vaccines must be kept ultracold or frozen while the Johnson & Johnson vaccine can be stored for months at refrigerator temperature.

There are also differences in how the vaccines work. The Pfizer and Moderna vaccines use an mRNA molecule to instruct cells to make copies of a harmless protein that is on the outer shell of the COVID-19 virus. When the immune system detects this protein, it begins to produce antibodies as if the body has been infected. The antibodies help the immune system fight off future COVID-19 infections. The Johnson & Johnson vaccine does the same thing, but it uses a DNA molecule instead of an mRNA molecule. None of the vaccines can give youCOVID-19, because they do not contain the coronavirus.

Pfizer is approved for patients 5 years of age and older. Moderna and Johnson & Johnson are approved for patients 18 years of age and older.

How effective is the Johnson & Johnson vaccine?

All three vaccines available in the U.S. currently are effective and safe.

The Johnson & Johnson vaccine completely prevented hospitalization and death in both the U.S. and in South Africa where a more transmissible variant of the virus is prevalent.

In clinical trials in the U.S., the vaccine was 85% effective at protecting against severe cases of illness and 72% effective at preventing moderate illness. While this falls short of the 94% efficacy rate seen in the Pfizer and Moderna vaccines, experts say head-to-head comparisons among the vaccines cannot be made, because the trials were conducted at different times during the pandemic and in different countries dealing with different variants and transmission rates.

Scientists also note that the Johnson & Johnson vaccine’s efficacy is far higher than the FDA’s 50% requirement.

Advisers to the CDC strongly endorsed the vaccine’s effectiveness after its approval.

How long does it take for the vaccine to work?

None of the vaccines provide immediate immunity to COVID-19.

The CDC estimates it will take “a few weeks” for immunity to take effect, no matter which vaccine is administered, however different people will reach peak immunity at different times. Johnson & Johnson recently reported that 90% of the participants in its clinical trial had coronavirus-neutralizing antibodies in their blood 29 days after immunization.

This is a faster average timeline than the Pfizer and Moderna vaccines, which take five and six weeks, respectively, to reach peak immunity after the first dose.

Should I wait to get my child vaccinated with another type of vaccine?

There is wide agreement among public health and medical professionals that people should not decline Johnson & Johnson’s vaccine in favor of other COVID-19 immunizations.

The Johnson & Johnson vaccine offers a high level of protection after just one dose, while Pfizer/BioNTech’s and Moderna’s products call for two doses.

Johnson & Johnson factored in some of the new, more contagious virus variants into its study, including the one first discovered in South Africa, which the Pfizer and Moderna trials did not originally include. It also studied more non-U.S. citizens than the other two candidates, and measured efficacy at different points in the study.

The CDC recommendation is to take whatever vaccine is available.  All three are preventing of severe disease, hospitalization and death.

How long will the vaccine protect my child from COVID-19?

Much like with the Pfizer and Moderna vaccines, it is not clear how long immunity from Johnson & Johnson’s product will last.

Scientists are trying to answer that question now. It is possible people might need annual shots, just like with the flu vaccine, especially as more variants emerge and become dominant.

If my child has already had COVID-19, should I still get them vaccinated?

Yes, your child should be vaccinated regardless of whether they already had COVID-19. That’s because experts do not yet know how long you are protected from getting sick again after recovering from COVID-19. Even if your child already had COVID-19 and has recovered, it is possible — although rare — that your child could be infected with the virus again.

Can my child stop wearing a mask once they have been vaccinated?

The CDC has said that it is okay for fully vaccinated individuals to stop wearing masks indoors and outdoors. You are considered fully vaccinated two weeks after receiving your second vaccine dose. However, this is an individual decision and certain factors, such as spending time around immunocompromised individuals or those at particularly high risk for severe disease, may make someone want to continue mask-wearing in those situations. Additionally, state, local and individual business regulations still apply until otherwise noted. If you are not yet fully vaccinated, you should not stop wearing a mask and should continue to practice safety measures like social distancing and frequent handwashing to minimize the risk of contracting COVID-19. The CDC has been updating its guidelines on mask-wearing as the vaccine rates rise. View the most recent guidelines on the CDC webpage.