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"It's still a race between the variants and the vaccine," said Amesh Adalja, a senior scholar at the John Hopkins Center for Health Security

The United States strategy to reach herd immunity through mass vaccination involves a delicate -- sometimes tricky -- dance with side effects, public opinion and virus variants.

All three are unpredictable, and can turn when you least expect it.

The rollout of coronavirus vaccines has not been without challenges, but the Biden administration's strategy of not relying on just one vaccine has limited the scope of a setback involving any one product. As this week's events illustrate, however, the suspension of a vaccine run the risk of affecting public opinion surrounding the entire process.

The Food and Drug Administration and the Centers for Disease Control and Prevention recommended a pause in the use of the Johnson & Johnson vaccine on Tuesday as they examine six severe cases of blood clots among the roughly 7 million people who have had the vaccine administered. J&J also halted its rollout in Europe.

The vaccine from J&J unit $(JNJ)$ Janssen is an adenovirus vector-based vaccine that only requires one shot. Clinical trials showed it had 72% efficacy in the U.S .

The two-shot mRNA-based vaccines made by Pfizer $(PFE)$ with German partner BioNTech SE (BNTX) and Moderna $(MRNA)$ make up the majority of shots administered in the U.S., and bith were about 95% effective in clinical trials. (Mayo Clinic research puts their "real world

Currently, 22.7% to discuss the cases, and the FDA has opened an investigation into the cause of the clots.

The J&J blood-clot issue is similar to one that caused many European countries to pause and/or restrict use of the AstraZeneca (AZN.LN) vaccine developed with Oxford University, which is also an adenovirus viral vector-based vaccine. The U.K. has restricted its use to those over age 30.

On Wednesday, Denmark said it would halt use of AstraZeneca. "The Danish Health Authority has decided to continue the vaccination against COVID-19 without the vaccine from AstraZeneca," it said in a statement. Other European countries have restricted its use.

"It's still a race between the variants and the vaccine," Amesh Adalja, a senior scholar at the John Hopkins Center for Health Security and a spokesman for the Infectious Diseases Society of America, told MarketWatch.

Operation Warp Speed, as Trump administration called its vaccine development and distribution program, has been key to this success, he said. "Part of Operation Warp Speed was not knowing which ones would cross the finish line, and having alternative vaccines that can handle the J&J pause and other vaccines in the pipeline," Adalja said. "We are increasingly not supply-constrained in the U.S. because of the bulk manufacture of vaccines."

The FDA and CDC said the J&J pause would give their scientists time to investigate the six cases of blood clotting in vaccinated individuals. Those cases of cerebral venous sinus thrombosis, a blood-clotting disorder, have emerged out of the 6.8 million--plus people in the U.S. who have received this vaccine.

All six cases of blood clots occurred in women ages 18 to 48. One woman died, and a second remains in critical condition, according to information released by the FDA. "We are recommending a pause in the use of this vaccine out of an abundance of caution," health officials said .

"Hiccups in production and hiccups in safety are inevitable," said Dr. Andrew Pavia, the George and Esther Gross Presidential Professor at the University of Utah and head of the Division of Pediatric Infectious Diseases. "It was a wise decision to spread the risk," he told MarketWatch. "Factories can be hit by a hurricane, run out of a supply, or be hit by contamination that forces them to shut down."

President Joe Biden, speaking to the media during an Oval Office meeting on Tuesday, emphasized that his administration has ordered 600 million doses of the mRNA vaccines from Pfizer and Moderna and thus can vaccinate all adults even without either the Johnson & Johnson vaccine or AstraZeneca's, which has not been authorized in the U.S. and has been paused in other countries, also over blood-clot concerns.

But the different efficacies of vaccines, their reported responses to variants and their respective side effects appear to take tolls on public opinion. "When I was offered Moderna, J&J wasn't even an option. It was never in my consideration not to take it," said Maury Newburger, a New York travel consultant who received the Moderna vaccine in March.

"Knowing what I know now, I probably would not take the J&J," he said. "I still think I would have taken the two-shot vaccines."

The good (and the bad) news

The good news: Pfizer-BioNTech and Moderna supply the majority of vaccines in the U.S., and currently ship roughly 23 million doses a week in the country. The White House said the J&J pause would not have a "significant impact" on the rollout in the U.S.

"We've been doing fairly well and not having the outcome Europe is having," Adalja said. "We have successfully vaccinated high-risk populations: nursing-home residents and those in community dwellings. We're nowhere near the winter surge. Nursing-home deaths have plummeted."

White House COVID-19 response coordinator Jeff Zients said in a statement : "This [J&J pause] announcement will not have a significant impact on our vaccination plan: [the] Johnson & Johnson vaccine makes up less than 5% of the recorded shots in arms in the United States to date."

The latest complication has further delayed a rocky rollout in the European Union, which ordered approximately 200 million doses of the J&J vaccine in 2021.

On Wednesday, the EU ordered 50 million more COVID-19 vaccine doses from Pfizer-BioNTech. "Those 50 million doses were initially foreseen for delivery in the fourth quarter of 2021. Now they are available in quarter 2," European Commission President Ursula von der Leyen said in a statement .

"We have made the decision to proactively delay the rollout of our vaccine in Europe," J&J said in a statement Tuesday. The U.K. has ordered 30 million doses of the J&J vaccine, although it has not yet been authorized for use there.

Now, for the bad news: "Unfortunately, there is always going to be a halo effect in a negative way," Dr. Aaron Glatt, who chairs the department of medicine at Mount Sinai South Nassau in Oceanside, N.Y., told MarketWatch. "What's happened with J&J is forcing people to have questions with all vaccines."

The J&J vaccine "remains an extremely important vaccine for a fatal disease," he added. "These types of things make vaccine-hesitant people more concerned."

"Certainly, having other vaccines has been extremely helpful because there can always be manufacturing issues, or different strains may or may not be effective against a particular vaccine. That's not intentional, it's just the way science works," Glatt said.

"The boosters, if and when they do come, will be more easily approved," he added. "We've done most of the legwork already. It's impossible to predict what will happen. It's possible the vaccines will have efficacy against different strains. Time will tell, and the different strains will tell as well."

In one recent Kaiser Family Foundation poll , taken before the J&J vaccination pause, 13% of Americans said they would definitely not get a vaccine, and 7% said they would get one only if required.

"As humans, we are not very good at translating risk into action," Pavia said. "If I have the chance of being one of the 500,000 who die of COVID, how do I balance that with the one person who had a fatal side effect, if it's a side effect at all? It's the same perceptual problem when we blindly drive to the airport texting, yet we worry about the airplane."

In the aftermath of the J&J vaccine pause, Barbara Alexander, president of the Infectious Diseases Society of America, said that the American public must continue to receive clear, accurate and up-to-date information, and have its questions answered, "so that we can maintain and build trust and confidence in COVID-19 vaccines."