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A single lot of the Moderna COVID-19 vaccine was investigated. Paul Hennessy/NurPhoto/Getty Images
  • Healthcare providers are allowed to again use a specific lot of the Moderna COVID-19 vaccine.
  • Providers had been told to hold off on using the lot after fewer than 10 people developed allergic reactions after getting the shot.
  • All people affected were successfully treated.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

Health officials in California have told healthcare providers they can resume using a single lot of the Moderna COVID-19 vaccine.

State officials had put a pause on using that lot of vaccine after six people in San Diego experienced severe allergic reactions after receiving the shot.

The people required medical attention after receiving the shot, according to a statement from the California Department of Public Health (CDPH).

More than 330,000 vaccine doses were distributed to 287 providers across California in early January.

Dr. Erica Pan, California state epidemiologist, said that the vaccines can start to be used immediately after the investigation.

“These findings should continue to give Californians confidence that vaccines are safe and effective, and that the systems put in place to ensure vaccine safety are rigorous and science-based,” Pan said in a statement this week. “I encourage every Californian to get the vaccine when it’s their turn.”

The lot contained a total of 1,272,200 doses, most of which were shipped across the country, according to a Moderna statement.

Thousands of those doses have already been administered.

Severe allergic reactions to vaccines are rare, with vaccine-related anaphylaxis occurring in approximately 1 in 100,000 people, according to the Centers for Disease Control and Prevention (CDC).

However, the vaccine is new, and health officials have been closely monitoring allergic reaction rates.

But out of an abundance of caution, the CDPH recommended that California providers administer other batches until the state, Moderna, CDC, and Food and Drug Administration (FDA) investigate the lot in question.

Now that the investigation has concluded, they’ve said the lot is safe to use again.

The CDPH ordered an investigation out of an abundance of caution.

“It is very normal for a pause on a lot to be ordered when there are more or unusual possible reactions,” said Dr. Anne Liu, an infectious disease physician at Stanford Health Care.

Liu said it’s tough to make any assumptions until data is available.

Though it would be highly unusual, there was a possibility the Moderna lot may have had manufacturing irregularities that other lots don’t contain.

“Moderna, unlike Pfizer, has outsourced its manufacturing because they are not a large company with a manufacturing infrastructure,” Liu said.

There’s also the possibility the cluster of reactions was a pure coincidence and is unrelated to the vaccines.

“Clustering of allergic reactions at a single vaccination site could be caused by other phenomena. For example, the healthcare workers may have all been preexposed to something in their community or work environment that made them more susceptible to an allergic reaction,” said Kathryn A. Whitehead, PhD, an associate professor of chemical and biomedical engineering at Carnegie Mellon University.

California has experienced a slow, delayed vaccine rollout over the past couple weeks.

Where some states, like West Virginia and North Dakota, have already used more than half of their doses, California has administered just 38 percent of what the state received.

The pause of the Moderna lot furthered delay the state’s vaccination efforts.

“I do not necessarily think it’s required to pause the lot,” said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security and an infectious disease expert.

Holding back doses during a critical time in the pandemic comes with risks.

And allergic reactions are easily recognizable and easily treated, said Adalja.

Liu, on the hand, thinks taking a break from administering the lot to investigate any abnormalities was the right decision.

“I agree with being cautious and pausing using this lot until more information can clear this up,” Liu said.

The San Diego community vaccine clinic reported a cluster of six allergic reactions potentially linked to the paused lot.

A woman in Colorado also experienced a reaction to a vaccine from the same lot.

“Thousands upon thousands of other doses of Moderna vaccine have been given without any such problems so far,” Liu said.

Other clinics will move forward with vaccinations using other Moderna lots.

Allergic reactions occur with every vaccine.

“The difference here is that allergic reactions to the SARS-CoV-2 vaccines constantly make the news, while allergic reactions to influenza, chickenpox, and hepatitis vaccines do not. The benefits of vaccination far outweigh the risk,” Whitehead said.

Whitehead said if she were offered a dose from the lot, she would gladly take it, assuming she would be monitored for the standard window after the shot.

All the allergic reactions reported in San Diego occurred within the standard observation window of 15 to 30 minutes.

Allergic reactions occur soon after vaccination. This is why people are advised to stay near the vaccination site for 15 minutes.

Allergic reactions are easily recognizable and treatable, said Adalja.

“As long as people are getting their vaccines in places where they are prepared to treat immediate allergic reactions, then they should still go ahead and get their vaccines,” Liu said.

If you’re worried about an upcoming vaccination appointment, you can ask your vaccination clinic whether the lot under investigation is on hold.

Anyone with a history of severe allergic reactions to vaccines should talk to their doctor before receiving the shot.

“This report should not dissuade people from getting what could be a lifesaving vaccine,” Adalja said.

California officials have allowed healthcare providers in the state to resume administering a batch of Moderna COVID-19 vaccine after a cluster of people in San Diego experienced allergic reactions after getting the shot.

The batch was investigated and found to be safe.

The risk of experiencing an allergic reaction after the vaccine is low, but the California Department of Public Health wanted to investigate the lot in question out of an abundance of caution.