Your guide to coronavirus and COVID-19

FREMONT, CA: More than 12.4 million doses of coronavirus vaccines have shipped to U.S. states, but just over 2.5 million people received shots as of Wednesday, according to the Centers for Disease Control and Prevention.

Officials say the pace of COVID-19 vaccinations should pick up significantly in the coming weeks. For now, however, they point to a host of reasons for the lag, including vaccination systems still gearing up, federal funding that hasn't yet been disbursed to states and conditions set aside vaccines for long-term-care facilities.

Two COVID-19vaccines are now authorized in the United States. The Moderna vaccine began arriving across the nation Monday, just three days after it was authorized for emergency use by the Food and Drug Administration. It comes on the heels of the Pfizer-BioNTech vaccine, which is being given to health care workers and nursing home residents.

Pfizer and the German biotechnology company BioNTech developed one of the COVID-19 vaccines that has been authorized by the FDA, BNT162b2. Moderna, a Cambridge, Massachusetts-based biotechnology company, developed a COVID-19 vaccine, mRNA-1273, that was authorized Friday.

All of the late-stage vaccine trials include at least 30,000 volunteers, half of whom receive the active vaccine and half the placebo. Both vaccines require two doses. The Pfizer-BioNTech shots are being given 21 days apart. Moderna’s are given 28 days apart. Beginning a week after the second dose, participants are watched to see whether they come down with COVID-19.

In each of the studies, after about 150 participants have developed COVID-19, it is statistically possible to determine the vaccine's effectiveness. Almost 200 trial participants developed symptomatic COVID-19 in the Moderna trial, only 11 of whom had received the active vaccine. Because the infection rate was so much higher in the placebo group, statistical analysis determined that the vaccine was 94% effective overall, according to safety and effectiveness data released Dec. 15.

Pfizer/BioNTech reported on Nov. 18 that of 170 confirmed cases of COVID-19 among its trial participants, 162 were in the placebo group versus eight in the vaccine group. A safety and effectiveness report released Dec. 8 confirmed the findings.

In Moderna’s Phase 3 trials, the company said the most common side effects were fatigue, muscle soreness and aches, joint pain and headache, plus pain, redness or swelling at the injection site.

In Pfizer/BioNTech Phase 3 trials, many participants endured side effects for a day or two after getting their shots, particularly the second one. The most commonly reported side effect among vaccine recipients under age 55 was a sore arm, followed by fatigue (60% after the second shot); headache (52% after the second shot); other muscle aches (37%); and chills (35%). About 28% took pain medication after the first shot and 45% after the second shot.

Most of the U.S.-backed vaccines target the “spike protein” found on the surface of the virus that causes COVID-19, which allows the virus to attach itself to host cells and infect them.

Both of these vaccines work by presenting this spike protein to the immune system. The spike proteins aren’t dangerous because the rest of the virus isn’t present; however, the body now sees the protein and designs immune "soldiers" to fight it.

The Moderna and Pfizer vaccines deliver strands of genetic material known as mRNA, which turns people’s cells into spike protein factories. Since the Pfizer/BioNTech vaccine has been approved, health care workers and people in long-term facilities across the country have been lining up to get their scheduled vaccine.

A few high-profile politicians, such as Vice President Mike Pence and Florida Sen. Marco Rubio, were publicly vaccinated in hopes of instilling confidence in the vaccine. A CDC advisory panel decided Sunday that police, firefighters, teachers, and grocery workers will be among the next in line for a COVID-19 vaccine.