It’s not a show-stopper, but the number of allergic reactions to the COVID-19 vaccine from Pfizer has definitely begun to raise eyebrows — epidemiologically, anyway. Operation Warp Speed’s top scientist said the frequency of allergic reactions is still incredibly small, but it has crept higher than one would expect from other vaccines. Pfizer might need more trials among higher-risk allergic populations, Dr. Moncef Slaoui suggested:

The frequency of allergic reactions to Pfizer’s Covid-19 vaccine is greater than what would be expected for other vaccines, Dr. Moncef Slaoui, chief scientific adviser for Operation Warp Speed said Wednesday. …

“That frequency, as it stood yesterday, is superior to what one would expect with other vaccines,” he said.

Slaoui said there are discussions between the vaccine makers and the National Institutes of Health to consider running clinical trials of Covid-19 vaccines in very allergic populations, such as those who have to carry an EpiPen with them at all times.

What exactly does it mean that the frequency is “superior”? How many serious reactions have been reported? As of Tuesday, Slaoui told CNN, the number stood at … six. But six out of how many?

Over a million and counting, which leads the world:

The U.S. now leads the world in Covid-19 vaccine shots administered, with 1,008,025 doses given in the 10 days since the first doses were cleared for use, according to the Centers for Disease Control and Prevention.

State-led vaccination campaigns are rolling out shots from Pfizer Inc. and Moderna Inc., focused at first on hundreds of thousands of health-care workers around the country who have been battling the virus on the front lines in hospitals. The vaccines require two shots weeks apart to deliver the highest level of protection.

China, which has several home-grown vaccines, has vaccinated more than 650,000 people and says it has administered 1 million doses. That figure could in fact be far higher, or at least higher than the U.S. total, since the country has provided infrequent updates on its effort. Russia has vaccinated 440,000 people. Both countries got a head start in August by authorizing domestically produced vaccines before they’d been fully tested.

Six out of a million? That’s pretty small frequency rate, which is why it didn’t come up in Phase 3 trials. That population would have been around 30,000 people in the active group; had that even produced one unexpected allergic reaction, we would have seen dozens or scores of serious allergic reactions by now. This is why the CDC and FDA have only granted emergency-use authorization to Pfizer and Moderna; they will require more trials to settle the questions remaining about impacts on specific sub-populations, especially as those emerge during the EUA vaccinations.

That’s no reason to stop now, however. Compare the allergic-response frequency seen thus far to the virus’ lethality rate. At its lowest estimate of 0.4% lethality (the flu is generally around 0.1%), you’d expect to see 4,000 deaths in a population of a million infections. Six serious allergic reactions simply doesn’t compare on a risk basis. An allergic reaction can be deadly, but clearly the numbers show it to still be an exceedingly rare event, and the shot a much better gamble than remaining unvaccinated.

Finally, some are complaining that the US is going to miss its target of 20 million vaccinations by the end of the month. That was always a bit aspirational, and logistics are proving complicated — as everyone should have known they would be. Getting one million vaccinations in the first ten days is a pretty amazing pace, though, and we only just added the second vaccine from Moderna three days ago. Once the states iron out the logistics and the priority rankings, the pace should pick up considerably. If it takes us until January 10 or so to hit 20 million, that’s still an amazing achievement.