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Home » COVID-19 vaccine safety signals – ACIP reports good news so far

COVID-19 vaccine safety signals – ACIP reports good news so far

The CDC’s Advisory Committee on Immunization Practices (ACIP) has reported that there are no COVID-19 vaccine safety signals through 16 February 2021 for the two vaccines available at that time from Moderna and Pfizer. This is good news and should allay the fears of those who might be on the fence about the vaccines.

I want to briefly examine what ACIP discussed regarding the COVID-19 safety signals. In addition, they looked at any concerns with regards to the vaccine and pregnant women – also good news. So, just good news.

COVID-19 vaccine safety

What are the COVID-19 vaccine safety signals?

ACIP reviewed data from three sources that track COVID-19 vaccine safe:

  • V-safe – a CDC smartphone app that allows individuals to check-in with health updates
  • Vaccine Adverse Event Reporting System (VAERS) – a self-reported system to file adverse events after any vaccination. I have long criticized VAERS because “VAERS reports alone cannot be used to determine if a vaccine caused or contributed to an adverse event or illness. The reports may contain information that is incomplete, inaccurate, coincidental, or unverifiable.” But it can rapidly detect safety signals.
  • Vaccine Safety Datalink (VSD) – a large database that includes several million medical records including vaccination status and health outcomes.

The three systems have provided a treasure trove of data about the COVID-19 vaccine over the past few months. Let’s take a look at the preliminary data on COVID-19 vaccine safety from each of these sources.


As of 16 February 2021, over 55 million people have received at least one dose of the Moderna or Pfizer COVID-19 vaccines in the USA. Of those, nearly 4 million utilized the V-safe app along with over 30,000 pregnant women.

For the first month of vaccination, from 14 December 2020 through 13 January 2021, no serious safety signals were observed.


Through 16 February 2021, there have been approximately 105,000 reports to VAERS for adverse reactions to the Moderna and Pfizer COVID-19 vaccines. Of those, about 94% were non-serious adverse events.

Most of the adverse events were minor, mostly related to the immune reaction to the vaccine. The investigators also could not establish a causal link between the vaccines and serious adverse events.

Since anaphylaxis had become a concern with both the Moderna and Pfizer vaccines, ACIP decided to drill down into this part of COVID-19 vaccine safety.

Researchers determine that there were approximately 4.7 anaphylaxis reports per million doses of the Pfizer vaccine and about 2.5 per million doses for the Moderna vaccine. This remains a very rare event that should be watched but it is not widespread.

Vaccine Safety Datalink

The statistically best way to perform epidemiological studies to establish not only correlation but also causation requires high-quality medical data that can be culled from actual medical records. VSD is a database that links vaccination status to medical records so that researchers can query whether COVID-19 vaccination status might be linked to something like Bell’s palsy.

VSD is far superior to VAERS or V-safe, which can’t establish correlation, let alone causation, between the COVID-19 vaccine and any safety issue. If a signal arises in VAERS, researchers can then query the VSD database to determine if that signal has shown up in a well-controlled analysis.

Nearly 630,000 individuals have received 1 dose of either vaccine, and over 200,000 have received both doses.

The researchers found no evidence of a statistically increased risk in any of the following adverse events when they compared the rate in vaccinated individuals compared to the background rate of the general population. Let me repeat that, there is no difference in the incidence of these adverse events in vaccinated and unvaccinated populations.

COVID-19 vaccine safety

However, because science is not dogmatic, researchers are going to continue to examine the database to include the following:

  • Dose-specific analyses – to determine if there’s a safety signal difference between one and two doses.
  • Product-specific analyses – to determine if there’s a difference between the Moderna, Pfizer, and, eventually, other vaccines like the recently approved one from Johnson & Johnson.
  • Analyses for two risk intervals 1-21 and 1-42 days – to determine if there are longer time period increases of safety signals

Based on the information from these three analyses of COVID-19 vaccine safety, ACIP found no issues that warranted concern, other than the issue with anaphylaxis which is still extremely rare.


Although the COVID-19 vaccines were considered safe for pregnant and lactating women, there was not a lot of data from the clinical trials to confirm that. The researchers decided to look at V-safe, VAERS, and VSD to determine if there were any safety signals for pregnant women.

Out of 1815 pregnant women who received the vaccine in the V-safe program, the researchers found the following:

COVID-19 vaccine safety

As you can see, the rates of the various outcomes were either lower than or statistically equivalent to the background rates of the outcomes. For example, in the V-safe registry, about 15% of women had a miscarriage <20 weeks. However, the background rate of miscarriage at that time point is around 26%.

No, I am not suggesting that the COVID-19 vaccine reduces the risk of miscarriage – however, if one observes a miscarriage soon after receiving the vaccine, that’s probably a result of random chance rather than the vaccine, because over 25% of unvaccinated women have a miscarriage.

VAERS shows a similar low level of adverse outcomes for pregnant women, but because we don’t have a method to determine how many are reported out of all COVID-19 vaccinations, it’s difficult to make a statistical comparison.

The researchers have not completed a review of the VSD database for pregnancy outcomes. As this issue will be monitored closely, we can expect a report within a few weeks.

COVID-19 vaccine safety


ACIP concluded the following from their analysis of the data:

  • Safety profiles of mRNA vaccines in V-safe monitoring are consistent with what was observed in clinical trials
  • Systemic and local reactions are most commonly reported to VAERS
  • Rare reports of anaphylaxis following both vaccines have been reported to VAERS
  • No other safety signals for serious adverse events have been detected in VAERS
  • No safety concerns have been identified through VSD Rapid Cycle Analysis as of February 13
  • No unexpected pregnancy or infant outcomes have been observed related to COVID-19 vaccination during pregnancy

I’m a scientist, so I’m not going to say that the COVID-19 vaccine is absolutely safe. But it is safe by any measure.

Almost all adverse events are local and minor. And serious adverse events do not appear to be any higher than what would be observed in a typical unvaccinated population. And the vaccine appears to be safe for pregnant women.

The CDC and FDA will continue to monitor the issue over the next months and years. And, if there are any signals that appear in the future, I’m sure we will hear about it, like how quickly the CDC dealt with the anaphylaxis concerns.


Michael Simpson

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