This Dec. 2, 2020 photo provided by Johnson & Johnson shows vials of the COVID-19 vaccine in the United States. The nation is poised to get a third vaccine against COVID-19, but health officials are concerned that at first glance the Johnson & Johnson shot may not be seen as equal to other options from Pfizer and Moderna. Credit: Johnson & Johnson via AP

A federal advisory panel recommended a third COVID-19 vaccine for emergency use in the U.S. on Friday, a development certain to accelerate the immunization effort in Maine and across the country.

The one-dose Johnson & Johnson vaccine has less strict storage needs than the two-dose Pfizer and Moderna vaccines in circulation. Those factors, along with a high number of doses expected out of the gate, will allow health officials to get the vaccine to a wider group of people. Final approval from the Food and Drug Administration is expected over the weekend.

The development could mean eligible people in Maine can get vaccinated in cars rather than doctor’s offices and other elements we have yet to see here. The company has said it will be able to provide 20 million doses nationwide by March’s end. Here is what you need to know.

How does the vaccine work?

The J&J vaccine is different from the other two vaccines in two crucial ways. It only needs one shot and it can be stored in a regular refrigerator for up to three months if necessary. Both the Pfizer and Moderna vaccine need to be kept at ultra-cold temperatures and need two doses administered three to four weeks apart to get the full immunization effect.

But it is also different in a key scientific way: It uses a viral vector method to create an immunological response by injecting a harmless virus — not the coronavirus — carrying DNA that teaches the cells to make copies of a spike protein found on the coronavirus.

Is this vaccine as safe and effective as the other options?

Yes, the FDA’s initial review of the vaccine has found it to be safe. Almost 50 percent of trial participants reported injection site pain when they received the vaccine, and adverse reactions such as headache, muscle pain and fatigue were reported in just over a third of the trial participants. About 14 percent experienced nausea and 9 percent experienced fever.

The median time it took for symptoms to set in was two days. Those side effects were mostly mild or moderate, however, and it is normal for recipients of any vaccine to experience some discomfort after being inoculated.

There have been a few numbers commonly mentioned when discussing the new vaccine’s efficacy. While coverage has noted lower effectiveness rates than the Pfizer and Moderna vaccines at preventing cases of the coronavirus, the most important one is that it proved to be 100 percent effective in preventing hospitalization and deaths in trials.

Studies in the U.S. found the Johnson & Johnson vaccine to be 72 percent effective at preventing moderate COVID-19 cases and 82 percent at preventing severe cases. It has also been found to protect against asymptomatic disease 74 percent of the time.

Those figures are not as high and the Pfizer and Moderna vaccines, which boast above-90 efficacy rates. But health experts caution against dismissing the vaccine on those grounds, because it still provides protection against the most severe cases of the disease.

How many doses can we expect and how will the vaccine be used in Maine?

Vaccines are distributed nationally based on a state’s adult population. The company’s estimate of 20 million doses by the end of March would equate to over 80,000 vaccines in Maine that month alone, greatly expanding the availability of doses. For example, the state administered less than 21,000 first doses of the other two vaccines in the past week as of Friday.

Maine Center for Disease Control and Prevention Director Nirav Shah said on Thursday the vaccine could mean more for the state than just increased supply, although that will be the main factor as demand for doses remains high. Shah said the vaccine’s versatility could create new opportunities to reach Mainers who might struggle to get the vaccine otherwise.

That could include drive-thru clinics — something Maine has hesitated to pursue for the Pfizer and Moderna vaccines because of the need for observation periods in case of reactions — and home visits for those in congregate settings. It could also mean giving vaccines to independent providers who have been left out of the vaccine efforts otherwise.