COVID-19 vaccines, irregular periods and spike protein shedding

Woman holding calendar looking confused

Are irregular periods a side effect of COVID-19 vaccines? Irregular periods include heavier bleeding than normal, early periods, late periods and similar changes.

Right now, there’s no scientific evidence that suggests COVID-19 vaccines are making periods irregular. 

“Some women have reported on social media that the period after a COVID-19 vaccine was different, or changed in some way from what they usually expect,” says OB-GYN Jennifer Griffin Miller, MD, MPH. “This was not identified in the clinical trials of the vaccines. There’s also no biological mechanism, based on how the vaccines work, that would explain these occurrences.”

Your period can fluctuate for many reasons, including diet, stress, exercise, illness and pregnancy. Nebraska Medicine OB-GYN Karen Carlson, MD, writes about how pandemic stress can affect periods. Oral contraceptives and other medications can also change periods, especially if they change hormone levels.

Abnormal periods will also happen by chance after people receive the COVID-19 vaccine. That doesn’t necessarily mean the vaccine caused the abnormal period. They could be related, but it’s too soon to say for sure.

“It’s not uncommon for women to experience an atypical cycle over the course of a year,” says Dr. Griffin-Miller. “When millions of menstruating women are receiving the vaccine, the timing could certainly be coincidence.”

Vaccines sometimes do create an immune response that you can feel. Often people report fevers, body aches, headaches and pain at the site of injection. These side effects often go away in a few days. See why it helps to schedule your mammogram around a vaccine.

If you’re experiencing persistently abnormal periods, seek care. Call 800.922.0000 to schedule an appointment with an OB-GYN who can help.

Can “vaccine shedding” cause side effects in unvaccinated people?

No. There have been rumors of “vaccine shedding” causing side effects to people who have not been vaccinated. The idea is that someone who has been vaccinated is shedding spike protein to those around them who have not been vaccinated. 

“We have no data to indicate that contact with somebody who has been vaccinated affects menstrual cycles,” says infectious diseases expert James Lawler, MD, MPH.

The vaccines can’t give you COVID-19. Vaccines do not contain SARS-CoV-2, the virus that causes COVID-19. And the spike protein itself can’t shed.

“Spike protein is primarily made locally in muscle where the vaccine is administered and may possibly be seen in low levels in the blood," says Dr. Lawler. "But it should not be shed in significant quantity in respiratory or other secretions.”

If someone has tested positive for COVID-19, though, they are shedding virus, including the spike protein, and contagious. “We know that people with COVID-19 shed large amounts of virus from respiratory secretions,” says Dr. Lawler.

Shedding can’t happen without a live vaccine. The mRNA vaccines – Pfizer and Moderna – are not live vaccines and do not replicate. The Johnson & Johnson and AstraZeneca vaccines are considered live vaccines because they both contain adenovirus. (Again, they do NOT contain the coronavirus.) But the adenovirus in both the Johnson & Johnson and AstraZeneca vaccines can’t replicate, so there’s no way they can shed. Learn more about how adenovirus vaccines work

But what about that Pfizer document?

Links to the clinical trial protocol document for phases 1, 2 and 3 of the Pfizer clinical trial are spreading on social media. There’s a lot of confusion about what the information in that document means. 
There are some fake versions floating around, so here is a link to the original document listed on
A clinical trial protocol is a document that describes the rules for a clinical trial. All clinical trials must have a clinical trial protocol before they begin. The protocol outlines the goal of the study, explains how it’s designed and provides instructions for the health care providers conducting the study. 
On page 67 of this document, there’s a section titled “Regulatory Reporting Requirements for SAEs.” SAEs stands for Serious Adverse Events. In this section, the document lists events that should be reported, including various types of exposure. “Exposure” just means that someone somehow came in contact with the substance being studied. And “study intervention” refers to the vaccine itself – the medicine (intervention) being studied. 
This document is not discussing viral shedding, or even indicating that there’s a potential for viral shedding. It also is not actually reporting adverse events. It is just telling health care providers how to document and track various types of potential adverse events. 
According to the data submitted to the U.S. Food and Drug Administration for review, 23 pregnancies were reported after the clinical trial began. There were 12 in the vaccine group and 11 in the placebo group. Two of these 23 women reported adverse events, but both were in the placebo group. 
Pregnant women are usually excluded from clinical trials. However, Pfizer began officially enrolling pregnant women into their ongoing COVID-19 vaccine clinical trial in February. More data will likely become available later this fall.