Like all new pharmaceuticals, the vaccines that have been authorized to protect against Covid-19 come with some safety concerns and side effects. Many people who’ve received the shots have experienced fever, headache and pain at the site of the injection. These side effects generally disappear quickly. A small number of recipients have had a serious, but treatable, allergic reaction, called anaphylaxis. Small numbers have experienced blood clots and others have had a form of temporary facial paralysis or weakness known as Bell’s palsy, but no connection to vaccines has been established. A few governments have reported deaths following administration of Covid-19 vaccines, but medical specialists have found no evidence the inoculations were to blame.

1. What’s known about the deaths?

• Norwegian officials were the first to report people dying after being inoculated, saying in mid-January that 33 people age 75 and older had died a short time after receiving the Covid vaccine developed by Pfizer Inc. and BioNTech SE. After a review, a committee of the World Health Organization said that the fatalities were “in line with the expected, all-cause mortality rates and causes of death in the sub-population of frail, elderly individuals.” The committee concluded that the risk-benefit balance of the vaccine “remains favorable in the elderly.”

• The U.K.’s Medicines and Healthcare Products Regulatory Agency reported that, through Feb. 28, there were 227 deaths shortly after injections with the Pfizer-BioNTech vaccine, 275 after the AstraZeneca Plc shot and four where the brand was unspecified. The deaths were mainly in elderly people or those with underlying illness, and there’s no suggestion the vaccines played a role, it said.

• In Germany, the Paul Ehrlich Institute said after an investigation that the deaths of seven elderly people shortly after receiving the Pfizer-BioNTech vaccine were probably due to the patients’ underlying diseases.

• In the U.S., which is using vaccines from Pfizer-BioNTech, Moderna Inc. and Johnson & Johnson, the Centers for Disease Control and Prevention said that as of March 8, there were 1,637 reported deaths among inoculated people, a rate of 0.002%, and no evidence suggests a link.

• Authorities in Hong Kong, which has mostly deployed a vaccine from China’s Sinovac Biotech Ltd., have reported six deaths among more than 150,000 people inoculated. None have been connected to vaccines.

2. What about the serious allergic reactions? What accounts for them?

The body fights foreign invaders through a variety of mechanisms that include making protective proteins called antibodies, releasing toxins that kill microbes, and marshaling guardian cells to battle the infection. As in any conflict, sometimes the effort to repel an infection can itself be damaging. In rare cases, it can produce runaway inflammation and swelling of tissues in a serious allergic reaction called anaphylaxis. As much as 5% of the U.S. population has had such a reaction to various substances. It can be fatal if, for example, the person’s airway swells shut, though deaths are rare. Allergies to insect stings and foods can provoke it, though drug reactions are the most common cause of anaphylaxis fatalities in the U.S. and U.K. As of January, no anaphylaxis-related deaths had been reported in the U.S. from either the Moderna or the Pfizer-BioNTech vaccines in use there, according to the CDC.

3. How often have Covid vaccines triggered cases?

In the U.S., according to the CDC, it occurs in just 2 to 5 people for every million receiving a Covid vaccine. The risk of contracting Covid outweighs that posed by the vaccines, officials and clinicians say. Anaphylaxis is a known risk of vaccination. Such reactions occur about 1.3 times per million doses of flu vaccine administered. With other vaccines they have been seen at rates of 12 to 25 per million doses, though the studies were small.

4. How long does the risk of allergic reaction last?

Usually not long. When anaphylaxis occurs, it is almost always within half an hour of administering the vaccine, according to the CDC.

5. What’s being done to manage the risk?

The U.K. and U.S. have advised people who have allergies to any component of a Covid vaccine not to receive it. Anaphylaxis can be quickly countered with antihistamines in tandem with adrenaline injectors like Mylan NV’s Epi-Pen that slow or halt immune reactions, and health workers giving the vaccine are keeping such items at the ready. These treatments don’t cancel out the beneficial effects of vaccines. In the U.S., health workers are observing everyone who receives the vaccine for at least 15 minutes post-injection to watch for signs of a reaction; those with a worrying history of allergic reaction are monitored for twice as long. People who have had reactions to a first dose of vaccine shouldn’t receive a second, according to the CDC.

6. Do we know what in the shots is causing the reactions?

That isn’t clear. Two leading candidates are polyethylene glycol -- a chemical found in many foods, cosmetics and medications -- and lipid nanoparticles that encapsulate the messenger RNA, a genetic component in the Moderna and Pfizer-BioNTech vaccines, according to Eric Topol, a clinical trials expert and director of the Scripps Research Translational Institute. Polyethylene glycol has been previously linked to a handful of anaphylaxis cases. Once a cause has been narrowed down, it may be possible to make Covid vaccines even safer than they are now, Topol said.

7. What’s known about the blood clots?

Several countries, mainly in Europe, suspended use either of all AstraZeneca’s Covid vaccine or doses from a particular batch following reports of blood clots in recipients, including one death in Austria and another in Denmark. The European Medicines Agency said it was investigating, but noted there was no indication of a connection to the vaccine. The company said on March 14 that the number of cases -- 37 out of 17 million shots administered -- was lower than what would be expected to occur naturally in a general population of that size. It also said that participants getting the vaccine in studies had fewer clots than those given placebo.

8. What about the cases of Bell’s palsy?

In studies testing the Moderna and Pfizer-BioNTech inoculations, more people developed Bell’s palsy, which typically affects just one side of the face, after receiving vaccine doses than placebo shots. The imbalance turned out not to be substantiated, however, based on the massive safety database the CDC has collected on millions of people vaccinated since the drugs were authorized for the general public. Also, the drugs regulator in the U.K., where the Pfizer-BioNTech formulation is one of two in use, noted that as of Feb. 28, the number of reports of Bell’s palsy among those vaccinated does not suggest an increased risk.

(Updates with additional deaths reported in Hong Kong in first section; Updated detail on clots in section 7.)

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