Vaccine providers in Colorado have now administered somewhere north of 4 million doses of coronavirus vaccine.

That’s a huge number, substantially larger than the number of flu vaccine doses given each season – though that vaccine typically requires a single dose as opposed to two doses for two of the three coronavirus vaccines.

It’s also a critical mass of data from which to study side effects.

With polls showing that possible side effects are one of the main reasons people are hesitant to get vaccinated, The Colorado Sun this week pulled federal data about reported side effects and spoke with experts about how to interpret it.

The results are both surprising and not. The large majority of reported side effects are mild. The ones that aren’t get special scrutiny – but there is also reason to be skeptical of whether they were actually caused by a vaccine, based on a quirk of the federal data-collection system.

Here’s what we found.

The Sun pulled data from the federal Vaccine Adverse Event Reporting System, or VAERS. It is a side-effect monitoring system jointly administered by the Food and Drug Administration and the Centers for Disease Control and Prevention.

“It’s sort of the nation’s early safety warning,” said Dr. Matthew Daley, a Colorado pediatrician with Kaiser Permanente who serves on the CDC’s prestigious Advisory Committee on Immunization Practices. “Anybody anywhere across the U.S. can report anything.”

As of this week, VAERS had received just more than 1,700 reports of coronavirus vaccine side effects in Colorado. Those reports documented more than 8,700 symptoms. (People can list multiple symptoms in a single report.)

The most common side effects were headache, chills, fever and fatigue.

The most commonly reported side effects for each of the three approved coronavirus vaccines – made by Pfizer, Moderna and Johnson & Johnson – all tracked pretty closely.

Headache was the top-reported side effect in both the Pfizer and Moderna vaccine. Chills were the most commonly reported side effect for the Johnson & Johnson vaccine, though that vaccine also accounted for the fewest reports of side effects among the three, indicative of its much less prominent role in the vaccination campaign so far.

With one exception, the top 10 reported side effects for each vaccine also were in the top 25 reported side effects for the other two vaccines. The only exception was for the Johnson & Johnson vaccine, where only a handful of people have reported redness in the area where the needle went in, a condition called injection site erythema. It is the second-most common side effect reported for the Moderna vaccine.

VAERS categorizes side effects as either serious or not serious. Serious side effects are those that might result in a hospital visit or worse. They get a follow-up investigation by health authorities to better understand them.

About 10% of the side effects reported for the coronavirus vaccines in Colorado have been labeled as serious.

There are some really serious reports in VAERS, but here it helps to understand the role of the reporting system.

As Daley said, VAERS is an open reporting system. Anybody can make a report to it – doctors and nurses, but also the people who received the vaccine or their family members. The purpose is to have one, big, catch-all system to collect reports about any possible side effect that may have come from a vaccine.

“What VAERS is going to be really good at is identifying anything that’s really unexpected and rare,” Daley said.

The flip side to its openness is that VAERS can contain reports for things that may have nothing to do with vaccination. For instance, there is one report in Colorado of “white coat hypertension” – which is elevated blood pressure caused by being nervous around doctors.

It’s also clear that the passive nature of VAERS – it only knows what people report to it – also misses lots of side effects. While clinical trial data showed that side effects were generally worse for the Pfizer and Moderna vaccines after their second doses, VAERS contains more reports for first-dose side effects for those two vaccines in Colorado than second-dose side effects.

What to make of the more serious reported side effects

Daley said this context is important in understanding the alarming-looking reports in VAERS.

The database contains 116 reports of hospitalization after coronavirus vaccination in Colorado. It contains 39 reports of death.

Sometimes, the underlying cause of death is not fully understood when a report is made. Other times, the report is made to VAERS out of an abundance of caution.

One of the reports of death, for instance, was for someone who died in a small plane crash the evening after his second dose. Another of the reports was for a man suffering from chronic kidney disease who died four days after his first dose of vaccine.

“Ultimately, we suspect that the patient’s condition was a direct result of his underlying disease states, but wanted to make sure reporting was made available,” the person filing the report wrote.

Daley said reports like these are to be expected for such an enormous vaccination campaign. Tragic as it is, dozens of people die every day in Colorado from all sorts of ailments. A vaccination campaign that has now reached more than 50% of adults in the state will likely vaccinate people who subsequently die for other reasons.

Once a report of a serious side effect pops up in VAERS, doctors at the CDC launch an investigation that includes looking through the patients’ medical records to determine if the vaccine is responsible. Those doctors will then look for similar side effects to determine if there is a pattern.

This is what happened when reports of rare blood clots began popping up – at a rate of about 2 per 1,000,000 vaccinations – related to the Johnson & Johnson vaccine. Federal authorities paused distribution of the vaccine while they investigated.

VAERS isn’t the only system authorities use to look for harmful side effects. There’s another CDC system called the Vaccine Safety Datalink, which automatically monitors health records of eight large health organizations, including Kaiser Permanente in Colorado. The U.S. Department of Veterans Affairs and the federal Medicare program also have monitoring systems, Daley said.

In Colorado, state health officials at the Colorado Department of Public Health and Environment receive weekly reports on VAERS data and might assist the CDC in investigating cases, if needed. But, it is primarily the federal government’s job to investigate vaccine side effects and tell states if there is a problem.

This is what is happening with the blood clots. While there are at least five reports in VAERS involving blood clots, including three potentially related to the Johnson & Johnson vaccine, the Colorado Department of Public Health and Environment says the CDC will ultimately determine if these events meet the definition of the specific type of clots that led to the Johnson & Johnson pause.

ACIP, the federal committee on which Daley sits, met last week to review data on the Johnson & Johnson clots. The committee ultimately recommended that the vaccine be put back into service, after concluding that the benefits of protecting people against COVID-19 far outweigh the risks posed by the clots.

Daley said it’s important to take the reports and investigations of potential side effects seriously. These are new vaccines, and it’s necessary to re-weigh the risks when new information is available.

But he said people also have to consider the dangers of the disease they’re trying to stop. While the clots are concerning – as is the risk of having a severe allergic reaction to the Moderna or Pfizer vaccines – they are rare occurrences.

“Your risk of dying from COVID is so much higher than that,” Daley said.

“My ultimate point,” he said, “is that given the severity of COVID, we’re really remarkably fortunate to have vaccines that have a high degree of safety.”

The Colorado Sun is a reader-supported, nonpartisan news organization dedicated to covering Colorado issues. To learn more, go to coloradosun.com.