With Friday’s emergency approval of Moderna’s coronavirus vaccine, Washington health officials now have a second tool in the fight against the virus, and it’s one that’s more flexible and should help vaccinate smaller communities farther from major hospitals.  

The Pfizer and Moderna vaccines were created using the same technology, report similar effectiveness and produce comparable side effects.

But key differences — Moderna is more temperature-stable, can ship in smaller packages and does not require dilution — will shape where public health officials direct these important doses of vaccine and who has access. 

“It’s just more manageable,” said Michele Roberts, the state Department of Health’s acting assistant secretary.

In its first week of shipments, the federal government plans to send out some 6 million doses of Moderna’s vaccine nationwide, Gen. Gustave Perna, the chief operating officer of federal vaccine program Operation Warp Speed, said in a Monday briefing.  

That’s more than twice as many doses as were planned for Pfizer’s first week, reflecting the additional time the government had to plan for shipments from Moderna, Perna said. 


Moderna’s vaccine was the first in the world to enter clinical trials, and Seattle was the inaugural site. The first volunteers were injected March 16 at Kaiser Permanente Washington Health Research Institute under the direction of senior investigator Dr. Lisa Jackson.

To see the vaccine gain emergency approval just nine months later is “incredible,” Jackson said.

“Now we can start fighting back and work on tamping down this raging epidemic.”

After initial trials found the vaccine was safe and elicited a strong immune response, nearly 200 people in the Seattle area joined almost 30,000 others at sites across the country to participate in the Phase 3 trials that showed the vaccine is 94.5% effective in preventing coronavirus infection.

The Pfizer-BioNTech vaccine that gained emergency approval Dec. 11 is 95% effective.

Numbers that high are almost unheard of in the vaccine world. 

“I would have never dared to hope for 95% effectiveness,” Jackson said.


But it’s not surprising results were so similar for the two vaccines because they both take the same approach to inducing immunity, she explained.

They rely on mass-produced snippets of genetic material, called messenger RNA (mRNA), that is the blueprint for the so-called spike protein that allows the novel coronavirus to latch onto and infect human cells.

The vaccines contain mRNA enveloped in tiny bubbles of lipid that can safely ferry the fragile cargo into the cells of a vaccinated person. The cells’ machinery then reads the blueprints and produces copies of the spike protein that embed themselves in the cell membranes.

The presence of the foreign proteins sets off alarm bells, and the immune system mounts a response that — hopefully — confers long-lasting protection against future infection.

“They want to mimic as closely as possible what the body sees or what the immune cells see when infected with an actual virus,” Jackson said.

The reason both vaccines need to be kept cold is that mRNA breaks down quickly.


Moderna has the advantage here because it was already working on another mRNA vaccine when the pandemic hit and had been experimenting with ways to make the formulation more heat-tolerant. Exactly how it accomplishes that is a trade secret.

“Each company has their own ways of making the synthetic mRNA and making the lipid capsule,” Jackson said. “So while they’re very similar, there are biochemical differences between the two.”

Both vaccines appear to work well across a range of age, racial and ethnic groups. The Pfizer-BioNTech vaccine is approved for people 16 and older, while 18 is the youngest age to qualify for Moderna’s.

Side effects for both vaccines were generally mild to moderate and went away within a day or two, but analysis by the FDA shows “adverse reactions” were more frequent — and sometimes more severe — with the Moderna shots.

Almost everyone who got the prescribed two doses of Moderna’s vaccine reported pain at the injection site. Nearly 69% reported fatigue, 63% had headaches, 45% had joint pain and 48% experienced chills. Nearly 16% developed more severe reactions, though they did not require hospitalization.

According to data from the Pfizer trials, 84% of participants had sore arms after the second dose, while 51% reported fatigue, 30% had headaches, 19% suffered joint pain and 23% had chills. Pfizer also reported a low rate of severe reactions, the most common being severe fatigue, which occurred in 4.6% of people aged 18 to 55.


Shipments are planned to 3,285 sites across the country, more than five times as many sites as for Pfizer’s first week. McKesson, a pharmaceutical distributor, will deliver Moderna’s vaccine, rather than UPS or FedEX, which are shipping for Pfizer.

Moderna’s broader first-week reach reflects more time, but also simpler logistics.

Moderna’s vaccine can be shipped in packages with as few as 100 doses. Each of its vials contains 10 doses. Pfizer’s vaccine comes in a pizza-sized box with at least 975 doses in 195 vials.

For smaller organizations or communities, that makes Moderna’s vaccine more appealing because they won’t need to partition out doses and send them elsewhere to avoid waste.

And whereas Pfizer’s vaccine requires dry ice or ultracold storage, Moderna’s can be kept at length in standard medical refrigerators. The vaccine remains stable at temperatures between 36 and 46 degrees Fahrenheit for 30 days and can remain at room temperature for 12 hours, according to a company news release.

Moderna’s vaccine does not require dilution at the vaccination site. 


With the Pfizer vaccine, “you have to have a pharmacy that has to know to manipulate it,” said Dr. Maria Elena Bottazzi, associate dean of the National School of Tropical Medicine at Baylor College of Medicine.

Dr. Jeff Duchin, health officer with Public Health — Seattle & King County, said it made sense to direct Moderna’s vaccine to health clinics and doctors’ offices, and for hospitals to primarily handle Pfizer’s product.

To use Pfizer’s product at the Seattle Indian Health Board, “we would have to have given 100 vaccinations a day,” said Esther Lucero, the organization’s chief executive officer. “We don’t have the capacity.”

The 200-employee health board expects to receive 500 doses of Moderna’s vaccine on Dec. 22, which it will use to immunize staffers and then community elders. 

Meanwhile, “we’re going to be a Pfizer location, specifically,” said Dr. Steve Pergam, medical director of infection prevention at Seattle Cancer Care Alliance (SCCA), which received 975 doses last Wednesday.

Ultracold freezers are commonly used in medical research, so storage is not a concern for Pergam. Between his projects, “I think I might have four, myself,” he said. 


SCCA began vaccinations last Thursday and Pergam hopes to vaccinate roughly 1,600 employees for their protection and to help protect the cancer patients they serve who are at higher risk of severe complications.

The U.S. has finalized the purchase of 200 million doses of Moderna’s vaccine and 100 million of Pfizer’s. Which one should people opt for? 

“Whichever they can get,” Kaiser Permanente’s Jackson said, adding that mixing and matching was not advised.

The Moderna and Pfizer vaccines probably won’t be the only games in town much longer. Trials on three other vaccines are underway in Seattle and across the country. Jackson is coordinating the local arm of the Phase 3 trial using Johnson & Johnson’s vaccine, which is unique among the major candidates in requiring only a single shot.

Results are expected soon — accelerated by the surge in infections across the country.