The Covid-19 vaccines are highly effective, and the chance of an adverse reaction is rare – CNN

But some developments have dampened the excitement: This week, a few vaccination sites across the US paused operations after some patients reported adverse reactions following the Johnson & Johnson vaccine. The vaccine has also been linked to blood clots in four people who received the vaccine. One of the cases was fatal, European health officials confirmed.

Incidents of "breakthrough infections" have caused some concern, too: A few people who are fully vaccinated against Covid-19 have still been infected with the virus and, in some cases, have died.

Both issues raise valid concerns, said Dr. William Schaffner, an infectious disease professor at the Vanderbilt University School of Medicine. But the risks of getting vaccinated -- severe reactions remain incredibly rare -- are outweighed by the risks of Covid-19 infection, he said.

Health experts in the US and beyond agree: All three Covid-19 vaccines currently authorized in the US continue to be safe and highly effective at preventing Covid-19 infection.

But 90% isn't 100%, so "breakthrough infections," or cases of Covid-19 that occur in fully vaccinated people, are to be expected and don't mean the vaccines are less effective, said Dr. Anthony Fauci at a White House briefing on Friday.

"That number of individuals who were breakthrough infections is not at all incompatible with a 90 plus percent vaccine efficacy. So I don't think that there needs to be concern about any shift or change in the efficacy of the vaccine," Fauci said in response to a question from CNN's Kaitlan Collins.

Johnson & Johnson is not a 'second-class' vaccine

The Johnson & Johnson vaccine was linked to four cases of blood clots, one case in a clinical trial and the remaining three during the vaccine's rollout in the US, the European Medicines Agency confirmed Friday. In one of the cases, the person died.

In a statement released on Friday, CDC spokesperson Kristen Nordlund told CNN, "CDC is aware of several incidents of vaccine recipients experiencing dizziness, light headedness, feeling faint, rapid breathing, and sweating (vasovagal or anxiety-related) symptoms following COVID-19 vaccines in Iowa, Colorado, Georgia, and North Carolina."

At this time, CDC and FDA "are not recommending health departments stop administering any lots of COVID-19 vaccine," the statement says. "CDC has performed vaccine lot analyses and has not found any reason for concern."

CDC officials take reports of adverse events seriously and "constantly analyze" the safety and effectiveness of vaccines, Schaffner said. Even considering those reports of adverse reactions, the Johnson & Johnson vaccine is still considered a safe and important tool to combat Covid-19.

Why some vaccinated people still get sick and even die

In Friday's briefing, Fauci noted that the few deaths seen among the 200-plus breakthrough infections predominantly occurred in elderly people. That's not unexpected, he said.

Older people are more likely than the rest of the population to have underlying conditions. Older people were also among the first to be vaccinated, in addition to health care workers, Schaffner noted.

Schaffner noted the fully vaccinated elderly are already at a heightened risk of death due to their age and health status.

"Those are populations that are enriched with people who have underlying illnesses," he said. "We know that, on a day-to-day basis, adverse events in that population are going to occur."

To determine whether the vaccine directly causes adverse reactions, health experts at the CDC and other agencies compare vaccinated people to unvaccinated people within the same demographics to find if adverse events occur more commonly in the vaccinated group. If they occur at similar rates, Schaffner said, the adverse events are likely not related to the vaccine.

Side effects aren't rare, but severe reactions are

Minor side effects after receiving a vaccine aren't uncommon. In the case of the Covid-19 vaccines, an estimated 10% to 15% of volunteers in clinical trials developed noticeable side effects, a former Operation Warp Speed official said late last year.

The risk from the Covid-19 vaccine is miniscule compared to the risk of severe illness from Covid-19 itself, Schaffner said. As case numbers surge once again in the US, even as millions of people are vaccinated, it's hugely important that those who can get vaccinated do, he said.

CORRECTION: This story has been updated to accurately characterize Dr. William Schaffner's comments about vaccine risks. He said the risk of Covid-19 infection outweighs the rare risk of adverse reactions to vaccines.

CNN's Jen Christensen, Katia Hetter, Betsy Klein, Amanda Watts and Holly Yan contributed to this report.

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The Covid-19 vaccines are highly effective, and the chance of an adverse reaction is rare - CNN