“A vaccine is designed to prevent a certain kind of thing — prevent an infection or prevent disease,” said Susan Ellenberg, a professor of biostatistics, medical ethics and health policy at the University of Pennsylvania’s Perelman School of Medicine. “It’s not going to prevent every bad thing that could possibly happen to anybody, so when a vaccine is widely used, all the other kinds of bad things that could happen to people are still going to happen.
“And when they happen in close proximity to getting a vaccine, one can understand why people think, ‘Oh, that must have been caused by the vaccine’ or at least be suspicious that it might have been caused by the vaccine,” Ellenberg continued.
Ellenberg and other experts emphasized that health conditions arising after a new vaccine or drug is distributed to the general population can be coincidental.
“Things that just happen by coincidence will, because it’s health-related, be assumed to be caused by the vaccine, but that is, in most instances, not the case,” said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine.
Here’s what scientists say you need to know about data on adverse effects after vaccination and why they, along with prominent health agencies and regulators, are continuing to support the use of authorized coronavirus vaccines during the pandemic.
Understand the numbers with context
There are two main ways researchers determine whether an adverse effect is related to a vaccine, Ellenberg said. “One is understanding the biology and how the vaccine might potentially cause this, and the other is understanding whether this is happening more frequently than might be expected in the general population.”
Although there isn’t exact epidemiological data on the prevalence of coagulation disorders, incidence estimates in 2017 ranged from about 300 per 100,000 people per year to about 650 per 100,000 people per year, according to a draft report requested by the European Medicines Agency that included data from several countries including the United Kingdom, Germany, Italy, Spain and Denmark.
In the case of AstraZeneca’s vaccine, the drugmaker said in a statement Sunday that out of more than 17 million people who were inoculated with the shots, there have been 15 events of deep vein thrombosis (DVT) and 22 events of pulmonary embolism reported as of early March. These reports have led to a number of countries, including Germany, France and Italy, to pause use of the vaccine, despite European regulators and the World Health Organization still maintaining that the benefits of AstraZeneca’s vaccine outweigh potential risks. (The vaccine has yet to be authorized in the United States.)
“Around 17 million people in the EU and UK have now received our vaccine, and the number of cases of blood clots reported in this group is lower than the hundreds of cases that would be expected among the general population,” Ann Taylor, AstraZeneca’s chief medical officer, said in the statement.
On Monday, the European Medicines Agency released a statement saying that while it is still investigating, “many thousands of people develop blood clots annually in the EU for different reasons. The number of thromboembolic events overall in vaccinated people seems not to be higher than that seen in the general population.” But during a news conference Tuesday, the agency noted that its review is focused on reports of very rare types of clots documented in Germany and elsewhere. German health officials have reported a handful of cases of cerebral venous sinus thrombosis (CVST), a blood clot that forms in the brain, among those who participated in the country’s vaccination program, which has administered 1.6 million doses of the AstraZeneca vaccine.
Still, Emer Cooke, head of the EMA, said the agency is “firmly convinced the benefit outweighs the risk” of side effects, adding that reports of the rare brain clots are a “serious concern and it does need serious scientific evaluation.”
“We are worried that there may be an effect on the trust in the vaccines, but our job is to make sure that the products that we authorize are safe,” Cooke said.
Meanwhile, the WHO said its advisory panel is also looking into reports of blood clots but noted that “there is no evidence that the incidents are caused by the vaccine,” according to Reuters.
“That to me says that it almost undoubtedly isn’t the vaccine,” he said. “I can’t be sure and I’m ready to change my mind on new data, but all the data I’ve seen says this isn’t a real problem.”
Correlation does not equal causation
Though the “temporal association is just too much for some people to ignore,” Ellenberg said it’s important to remember that there are a number of reasons people may develop adverse health conditions post-vaccination, particularly those such as common blood clots.
“It seems like an incredible coincidence,” she said. “But when you’ve got millions and millions of people receiving the vaccine, you know, coincidences happen.”
The challenge for scientists is trying to figure out “whether it’s a real signal or whether it’s just noise,” said Andrew Stolbach, an associate professor of emergency medicine at Johns Hopkins University and an expert on drug reactions.
“The public should be reassured that there are people that follow all these events that are checking to see if they’re related,” Stolbach said. “If you’re in the public, you should remember that things that make the news are always going to be the worst things and the most shocking things and the most scary things and the most exciting things, but that doesn’t mean there’s a relationship.”
In general, older people may have a greater chance of being at risk of developing blood clots, as well as those who are taking certain medications, such as oral contraceptives, Evans said. To determine a causal link between the AstraZeneca vaccine and blood clots, experts said, an epidemiological study would be needed.
“You will almost never base a judgment only on reports, whether it’s to the [U.S. Food and Drug Administration] or the U.K. agency or to other agencies around the world, because the reporting of things can depend on fashion, depend on anxiety, level of scare, other issues around the vaccine,” Evans said.
Evans said he is confident that the necessary epidemiological research can be conducted “quite rapidly” in the U.K. In the meantime, he urged people who are in areas where there are high rates of transmission to take the AstraZeneca vaccine if it is offered to them.
Evans, who is 77, said he has received two doses of the vaccine and did not experience any serious adverse effects.
The current concerns surrounding the vaccine are an example of “anecdotes being turned into data, which is not what epidemiologists deal with,” he said.
Stolbach recommended staying up-to-date on the latest science and looking to regulatory agencies and public health authorities for guidance. People, he said, should “keep an eye on the big picture instead of getting worried about all the individual scary cases that they hear about.”
“The thing that we’re trying to prevent, covid, is harmful itself,” he said, “so when we’re weighing the potential risk of a vaccine, we have to weigh it against the risk of not being vaccinated.”
Loveday Morris contributed to this report.