Short-Term Side Effects of the COVID-19 Vaccine During Lactation
by Anne Eglash MD, IBCLC, FABM
Lactating individuals have been sharing their side-effect experiences from the COVID-19 vaccines on social media, but what is the true incidence of these reported side effects, such as drop in milk production, or mastitis?
The study for this week, from the University of Washington, recruited approximately 17,500 individuals; 7809 were pregnant, 6815 lactating, and 2901 planning pregnancy at the time of their first COVID-19 vaccination. Most of the participants were from the USA, identified as Caucasian and worked in healthcare, with 62% receiving the Pfizer vaccine, 37.8% the Moderna vaccine, and 0.3% the Janssen vaccine.
Among all participants, 97% reported vaccine reactions, with the most common reaction being pain at the injection site and fatigue. For all 3 participant groups, side effects including fatigue, body aches, headache, chills and fever were worse after the second vaccine than the first. All side effects other than pain at the injection site, for both doses of vaccines, were less common in pregnant than in lactating or preconception participants. The authors concluded that the COVID-19 vaccines were well tolerated in pregnant, lactating, and preconception people.
But, what about lactation-specific side effects? Check out the question!
- This study found an approximate 2% risk of mastitis among those who were lactating.
- Approximately 5% and 7% of lactating participants reported decreased milk supply for 24 hours after the first and second vaccines, respectively.
- Approximately 10% and 12% of lactating participants reported decreased milk supply for 24 hours after the first and second vaccines, respectively.
- Approximately 2% reported ‘interrupted breastfeeding’ after the first and second vaccines.
- Approximately 3-4% of lactating participants reported concerns about their infants after each vaccine.
See the Answer
Vaccines against SARS-CoV-2 are highly effective in preventing COVID-19 illness. Research has found that COVID-19 is associated with adverse events in pregnancy, and recommendations therefore include offering SARS-CoV-2 vaccines to pregnant and lactating individuals, despite their lack of inclusion in initial clinical trials. To date, limited data on vaccine and pregnancy outcomes exist for SARS-CoV-2 vaccines in pregnancy and lactation The objective of this study was to investigate experiences of pregnant and lactating individuals after receiving COVID-19 vaccines.
In January 2021, we launched an online prospective cohort study of adults primarily located in the United States who were pregnant, lactating, or planning pregnancy at the time of COVID-19 vaccination. This study was determined to be exempt from institutional review board review by the University of Washington (UW) Human Subjects Division (Common Rule category 2). We followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline. Individuals who were recruited and enrolled online in the UW COVID-19 Vaccine in Pregnancy and Lactation Registry using chain-referral and snowball sampling were invited to participate. Electronic written informed consent was obtained, and self-reported demographic (including race and ethnicity), pregnancy, vaccination perception, and outcome data, including report of day 1 vaccine reactions, were entered via surveys into Research Electronic Data Capture (REDCap) software version 11.1.2 2021 (Vanderbilt University). Race and ethnicity data were collected to report diversity representation within our study population. Options for race and ethnicity were outlined following the Centers for Disease Control and Prevention’s National Health Interview Survey race and ethnicity categories. We performed statistical analysis (ie, χ2 tests and 1-way analysis of variance, odds ratios [ORs], and 95% CIs) using Stata statistical software version 16.1 (StataCorp). An α level of P ≤ .05, 2-sided, denoted significance. Data were analyzed from January through March 2021.
As of March 16, 2021, 17 525 individuals (17 364 [99.7%] women among 17 418 individuals with sex data; mean [SD] age, 33.6 [3.6] years among 17 518 individuals with age data; 15 361 White individuals [87.6%] among all individuals) with known pregnancy status receiving at least 1 dose of a COVID-19 vaccine had enrolled in the study. Owing to missing data, percentages for participant characteristics are among those with data for that variable. There were 3 distinct groups: 7809 individuals who were pregnant (44.6%), 6815 individuals who were lactating (38.7%), and 2901 individuals who were neither pregnant nor lactating but planning pregnancy in the near future (16.5%) at the time of their first vaccine dose. Most individuals received the Pfizer-BioNTech BNT162b2 vaccine (10 790 of 17 431 individuals [61.9%] with data on vaccine type) or Moderna mRNA-1273 vaccine (6592 individuals [37.8%]). Most participants resided in the United States, were employed in health care, and had completed some college education (Table). Among all participants, 15 055 individuals (85.9%) reported receiving 2 doses.
Among all participants, 17 005 individuals (97.0%) reported any postvaccination reactions after the first dose, with the most common reactions being pain at injection site (16 019 individuals [91.4%]) and fatigue (5489 individuals [31.3%]). The frequency of reactions after the second dose was higher than after the first dose (eg, 10 399 individuals [69.2%] with fatigue after the second dose), but with similar distribution of symptoms (Figure). Odds of several reactions were statistically significantly decreased among individuals who were pregnant (eg, fever after BNT162b2 dose 2: OR, 0.44; 95% CI, 0.38-0.52; P < .001 and after mRNA-1273 dose 2: OR, 0.48; 95% CI, 0.40-0.57; P < .001) compared with individuals who were neither pregnant nor lactating (Figure). Mean (SD) maximum self-reported temperature was 38.1 (0.6) °C (100.6 [1.0] °F) among 499 participants with fever after dose 1 (including 131 pregnant individuals) and 38.2 (0.6) °C (100.7 [1.0] °F) among 3293 participants with fever after dose 2 (including 1051 pregnant individuals). Participants seeking medical care after vaccination included 100 individuals (0.6%) after dose 1 (including 50 pregnant individuals) and 221 individuals (1.5%) after dose 2 (including 156 pregnant individuals).
Among pregnant participants, any obstetrical symptoms were reported by 346 of 7809 individuals (4.4%) after the first dose and 484 of 6444 individuals (7.5%) after the second dose. Altogether, 6586 pregnant individuals (84.3%) had reported a second vaccine dose at the time of data analysis. Of these, 6244 individuals (94.8%) were still pregnant, while 288 individuals (4.3%) had delivered and 49 individuals (0.7%) reported miscarriages at the time of their second vaccine dose. Among lactating individuals, interrupted breastfeeding after vaccination was reported by 155 of 6815 individuals after the first dose (2.3%) and 130 of 6056 individuals after the second dose (2.2%), decreased milk supply for less than 24 hours by 339 individuals after the first dose (5.0%) and 434 individuals after the second dose (7.2%), and concerns about the infant after vaccination by 208 individuals after the first dose (3.0%) and 267 individuals after the second dose (4.4%).
This large prospective cohort study found that COVID-19 vaccines were well-tolerated among individuals who were pregnant, lactating, or planning pregnancy. A strength of this study was the ability to compare vaccine reactions and perceptions in pregnant and lactating individuals vs individuals of similar age and fertility intentions who were neither pregnant nor lactating. Vaccination reactions for day 1 were similar among groups and comparable with findings among pregnant individuals previously reported. All groups reported increased reactions following dose 2 of BNT162b2 and mRNA-1273 vaccines.
Study limitations include that participants were drawn from a convenience sample with self-reported reactions and with limited perinatal outcome assessment, reflecting the first wave of vaccination, which largely consisted of health care workers owing to vaccine eligibility at the time of this ongoing study. As a result, our findings may be biased and not generalizable to all populations. In addition, there is potential participant overlap between our study and similar studies. Further studies are ongoing to investigate outcomes after receipt of COVID-19 vaccines among pregnant and lactating individuals.
The researchers did not report on incidence of mastitis after the COVID-19 vaccine among the lactating participants. They did not define what is meant by ‘interruption of breastfeeding’ after the vaccine, or the specific infant concerns after receiving the vaccines.
The report of decreased milk production after the vaccine is noteworthy, 5% after the first vaccine and 7% after the second. My own patients have reported this to me as a short-lived side effect. Possible reasons could include decreased feeding frequency when feeling ill, fluid losses from fever/poor oral intake, or an elevation in cortisol, which can decrease prolactin.
Another recent study was published in Breastfeeding Medicine, June 2021, surveying 4,455 breastfeeding mothers who underwent one or more doses of the COVID-19 vaccine. This group was similar, as they were predominantly white, educated, and worked in healthcare. This study also reported a higher rate of generalized postvaccine symptoms after the second vaccine than the first. Regarding effect on milk production, 4% of mothers reported an increase, and 6% reported a decrease. Interestingly they found that those reporting adverse lactation effect(s) were more likely to have side effects of the vaccine such as fever, chills, headache, pain at the injection site, and body aches. Only 0.1-0.2% participants reported mastitis after either the first or second vaccine.