Many people are worried that they may not receive their second dose of the vaccine on schedule because of delayed deliveries of the vaccine, or because of suggestions that the scheduled second doses of the vaccines should be given as a first dose to others so that more people can be immunized.

Currently, a second vaccine dose is recommended for the Pfizer vaccine at 21 days after the first dose, and for the Moderna vaccine at 28 days. We don’t have exact data on these new vaccines, but based on experience with other vaccines, the second doses are expected to increase your immunity even if they are given up to years after the first immunizations (“booster shots”).

When you are first exposed to a virus, your immune system has to learn to respond to a new virus and you get only a small increase in cells and cytokine chemicals to battle the infection. The next time that you are exposed to that virus, you get a much higher immune response, whether it is three weeks, six weeks, or even more than a year later.

The current recommendations of 21 and 28 days are based on the data gathered during the trials of the Pfizer and Moderna vaccines. The CDC now recommends, “if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna Covid-19 vaccines may be scheduled for administration up to six weeks (42 days) after the first dose.” You would probably still get a booster immune response even with a second dose given a long time after six weeks.

Walgreens Pharmacy currently plans to give immunizations at its stores throughout the country and says that, “patients can receive the second dose as long as it follows the immunization window.” That means that the second dose will be given at or after the scheduled time frame of 21 or 28 days, so the current recommendation is to try to get your second dose on schedule, but if that is not possible, you will still get a booster immune response whenever you receive your second dose.