Johnson & Johnson vaccine rollout paused in the United States. Why baseline rates are everything

The bad news is that in the United States, 6 women ages 18-48 appear to have developed a rare but serious blood clotting disorder after receiving the Johnson & Johnson vaccine. One died and another is still fighting for her life. The good news is that around 1 million women in that age range received the vaccine without incident, saving many, many lives in the process. Even if more cases of this apparently vaccine-induced condition are found—which is likely—we need to watch out for something very important: proper comparisons.

Today, many experts floated a variety of numbers about blood clots online and on television. This can be helpful but may also lead us astray. First of all, we need to understand the baseline rates of abnormal blood clots, in order to distinguish signal from noise. For example, in the Johnson & Johnson trial, around 1 in 2000 people reported that abnormal clots had developed; the punchline is that this occurred in the placebo group. Around 1 in 1700 people who received the vaccine also reported such clots. The difference was not statistically different. The fact that it took millions of doses in both the US and Europe for a handful of these more serious cases (which cause a kind of stroke) to emerge suggests that clinically relevant clots are rare indeed. But I actually assume that there are very likely many more clots in both arms of the Johnson & Johnson trial than we will ever know about. In fact, in a sense there are many that we should not even care about. Why? Because most of the undiscovered clots were so mild (or asymptomatic) that they were clinically irrelevant. (By clinically irrelevant, I mean that they did not actually require treatment, and led to little or no symptoms, and no long-term effects were caused.) 

There are two key things to know about blood clots. First, not all blood clots are created equal. Some cause no symptoms at all and pose no risk. In fact, treating these clots with blood thinners may do more harm than good and may not be warranted. Second, the more clinicians look for blood clots in dangerous places like the legs, lungs, and brain, the more of them you find. However, clinicians rarely find important ones that were not already highly suspected. This means that when doctors and other healthcare providers are over-zealous in testing for blood clots, they frequently find blood clots that resemble danger, but are not actually dangerous. I worry that in the coming days and weeks, the rate of blood clots among Johnson & Johnson vaccine recipients will increase substantially. It's likely, however, that most of these clots will not be dangerous. What we really care about is how many dangerous clots occurred, like the ones in the brain found in many of the patients in the AstraZeneca/Oxford studies published last week and the Johnson & Johnson reports. The same is true for most blood clots caused by oral contraceptive pills (OCPs) made of hormones. The rate of blood clots sounds pretty high among women taking hormonal OCPs, (my friend Dr. Angela Rasmussen, who I respect greatly, tweeted a statistic that one in 3,000 women on OCPs develops abnormal blood clots). But the number of these clots that are truly dangerous events is far lower, in actuality. In the coming weeks, we need to make sure we are using the same power microscope, metaphorically speaking, to compare the rates of blood clots related to the Johnson & Johnson vaccine to other more common causes. After all, what matters is not just how often these problems occur, but how often they have any meaningful effect on those who develop them. If a far higher number of people are found to have developed vaccine-related blood clots than currently suspected, the larger question will be just how many of these events were truly dangerous.

Ultimately, the most important decision to be made is not whether or not to receive a coronavirus vaccine. It's whether to receive a coronavirus vaccine or covid-19. To make that risk-benefit calculation, what matters is the rate of serious covid-19 by age and sex (which we generally know) and the rate of serious vaccine-induced blood clotting problems by age and sex (which we are just starting to study). Once we know the answer to that question, we can safely decide how to proceed. But if we ask the wrong question, we are consigned to a guaranteed wrong answer.

US approaching full vaccine eligibility

Last week President Biden pushed the deadline for vaccinating all Americans forward, a landmark date in the ongoing struggle to overcome the pandemic. Initially set for May 1st, he has now targeted April 17th, given the success of having vaccinated one hundred and sixty million Americans along with many doses waiting to be shot into arms.

Recall that while the National Academies of Science, Engineering, and Medicine (NASEM) published their recommended eligibility schedule as the vaccine candidates were still applying for authorization, it was left to the states to determine their own distribution plans, which led to regional inconsistencies due to differing timelines.

Many states have had an accelerated plan that has allowed them to offer vaccines to everyone sixteen years and older, though such efforts have not been entirely successful. A recent report covered by Brief19 showed that the number of doses could not keep up with the expanded pool, resulting in regional shortages that largely mitigated any progression to wider eligibility. The people who have suffered the most when this happens are the most at-risk, like those in the correctional system. With supplies stretched thin, there is no clear solution about how to ensure vulnerable populations don't get left behind. Various.

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