That can breed some confusion. Do you have to wait three weeks or four weeks between doses? How long after your doses are you truly immune? How many shots do you even have to get, depending on which vaccine you’re getting? And why are there differences and irregularities across nations on the timing between shots?

Pfizer/BioNTech

Pfizer and BioNTech’s highly effective COVID vaccine is of the mRNA variety, as is Moderna’s. These are the first products using a so-called messenger RNA platform to be rolled out to wide swaths of the public.

Vaccines are designed to get your body to replicate large numbers of antigens (the biological materials which can induce an immune response to an unknown pathogen) over time. With traditional vaccines, that usually involves injecting dead or weakened versions of the pathogen into your body, to kickstart the immune response.

But mRNA vaccines essentially take the process a step back: They deliver a set of instructions to your own cells, training those cells to generate an immune response. In the case of COVID, that response focuses on the coronavirus’s most vulnerable element, the so-called spike protein that it uses to hook on to your cells.

For maximal effectiveness, you require two doses of these vaccines, although there is evidence that even one dose of the Pfizer or Moderna vaccines provides significant protection. It also takes about two weeks after your final dose before your immune system has produced enough antibodies to be protective to yourself and others.

For the Pfizer COVID vaccine, you’re supposed to wait three weeks between the first and second dose, meaning the entire process, from first dose to immunity, lasts about five weeks. (Note: Even after that, public health experts urge vaccinated people to keep wearing masks and socially distancing in public, and following basic public health precautions, since variant coronavirus strains are a growing problem.)

Moderna

Although Moderna’s COVID vaccine is based on the same underlying mRNA approach as Pfizer’s, its timeline is a bit different. Rather than three weeks, the dosage schedule is four weeks between the first and second shot for the Moderna vaccine. That’s because while the technologies are similar, the vaccines are still comprised of different biological elements.

But that’s not the whole story, which highlights the evolving nature of our understanding of these brand new vaccines.

“The recommended interval between doses is 21 days for Pfizer-BioNTech and 28 days for Moderna; however, up to 42 days between doses is permissible when a delay is unavoidable,” writes the Centers for Disease Control (CDC). Longer waits have taken place in some jurisdictions where supplies for second doses were scarce, including in Pennsylvania earlier this year.

That means that, based on availability, it could take up to eight weeks to receive both of your vaccine doses and then give your body two weeks’ time to develop the coronavirus antibodies.

Johnson & Johnson

Johnson & Johnson’s COVID vaccine is of the single-shot variety. It’s still recommended to wait two weeks after the injection for antibodies to build up.

The topline efficacy number for J&J’s vaccine isn’t as impressive as the Pfizer and Moderna ones, but it is still remarkably effective and more convenient since it only requires a single dose.

The company also says that it’s working on developing booster shots (as are Pfizer and Moderna) to tackle evolving strains of the coronavirus should they become a problem in the future. The same two-week timeline between getting a J&J shot and building up antibodies will likely still be the recommendation.

AstraZeneca

AstraZeneca’s COVID vaccine, a more conventional type than Pfizer’s or Moderna’s, has been the subject of significant controversy in recent weeks over concerns over data in the company’s efficacy studies and bungled communication with regulators. Still, they are being deployed internationally (though not yet in the U.S.)

In clinical trials, doses of AstraZeneca’s vaccine were originally scheduled 4 to 12 weeks apart across various studies. But during the rollout, recommended waits between doses have varied by country.

For instance, the U.K. implemented a 12-week gap between the two doses in the AstraZeneca regimen, instigating a major scientific debate. It’s a balance between managing the supply of available doses, which have been administered to narrower swaths of the population than in the U.S., and sussing out whether or not such a long delay helps or hurts building up the immune response to COVID.

In other European nations, such as Italy and Germany, the gap between doses may be even greater. As the continent struggles with a supply chain dilemma, some providers have pointed to studies suggesting that a longer wait between the two doses may be beneficial, and others have flagged research suggesting that a single dose may be enough for those who had been infected with COVID-19 up to six months before getting their first shot.

A pooled analysis published in the journal Lancet in late February suggests there isn’t much of a deleterious effect on COVID antibodies with a longer gap between the first and the booster dose of AstraZeneca’s vaccine, with preliminary evidence suggesting it actually may be a more effective biological strategy than a shorter time between shots.

More information will clearly be coming in on this issue via ongoing studies. But regardless of which vaccine you’re getting, Pfizer’s, Moderna’s, Johnson & Johnson’s, or AstraZeneca’s, consult with your physician on the dosing timeline, since it can be so variable. And be prepared to wait up to several months before all those COVID antibodies form.