COVID-19 Vaccination During Breastfeeding and Its Possible Negative Effect on Milk Production and Supply: A Preliminary Observation

Despite high vaccine efficacy, COVID-19 vaccine willingness among lactating women has been reported to be rather low. ¹ Of interest, several articles reporting on the safety of COVID-19 vaccines during breastfeeding have noted a reduction in milk supply after vaccination. For example, the number of women affected by decreased milk supply ranged between 5% after a first dose and 23% after a second dose,^2–4 with the milk production generally returning to normal within 24–72 hours.^2,3 Owing to the sparse information on this topic as yet, we aimed to provide more systematically collected information regarding the incidence of decreased milk supply after COVID-19 vaccination.

This report is based on spontaneous data collected by the Teratology Information Service, part of the Netherlands Pharmacovigilance Centre Lareb. Data collected between January 1, 2021, and December 31, 2021, were assessed and quantitatively analyzed using descriptive statistics. Reporters consented to data storage and usage.

In total, Lareb received 194 reports of reduced milk production after COVID-19 vaccination during the study period (Table 1). The total number of live births in the Netherlands in 2020 was approximately 170,000 (no data for 2021 are available yet). ⁵ According to official statistics, 4.3 million COVID-19 vaccine doses were administered in 2021 to women aged 18–45 years and living in the Netherlands (RIVM, 2022, Personal Communication).

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Table 1.

Detailed Overview of the Spontaneous Reports of Decreased Milk Supply After COVID-19 Vaccination Collected by the Netherlands Pharmacovigilance Centre Lareb

	N	%
Reported vaccine (N = 194)
BioNTech/Pfizer	172	88.7
Moderna	16	8.2
AstraZeneca	3	1.5
Janssen	3	1.5
Systemic side effects reported (N = 194)
Yes	125	64.4
No	69	35.6
Type of systemic side effects reported a (N = 194)
Headache	61	31.4
Fatigue	57	29.4
Myalgia	51	26.3
Malaise	37	19.1
Fever	30	15.5
Chills	28	14.4
Arthralgia	19	9.8
Nausea	16	8.2
Latency (days) (N = 188)
<1	46	24.5
1–2	116	61.7
3–7	17	9.0
>7	9	4.8
Duration of the reaction (days) (N = 65)
1–2	9	13.8
3–6	33	50.8
7–10	15	23.1
11–14	8	12.3
Self-reported milk production after vaccination compared with the prevaccination amount (%) (N = 74)
<25	32	43.2
25–50	33	44.6
50–75	9	12.2
75–100	0	0
Burden (N = 189)
None at all (1)	1	0.5
A little (2)	18	9.5
Quite (3)	49	25.9
A lot (4)	52	27.5
Very much (5)	69	36.5
Age of the child at the time of the suppressed lactation (weeks) (N = 99)
0–4	22	22.2
5–8	32	32.3
9–12	21	21.2
13–16	6	6.1
17–20	4	4.0
21–24	1	1.0
>24	13	13.1

Results are presented as absolute numbers and %.

a

The systemic side effects reported by ≥10 cases are presented.

Most women received a BioNTech/Pfizer vaccine (88.7%). In 125 reports (64.4%), maternal systemic side effects were reported in addition to a reduction in milk supply. Headache, fatigue, myalgia, malaise, and fever were the most reported reactions.

The median latency time of reduced milk production was 1 day (IQR = 1). Twenty women (10.3%) reported full breastfeeding cessation. In the cases of full cessation, the median age of the infants was 5.25 weeks (IQR = 6), compared with 8 weeks (IQR = 7) in the overall cohort. In 65 of 74 cases with available information (87.8%), a decrease in milk production to <50% of the prevaccination amount was reported, which was objectively confirmed in 73% of the cases by a reduced amount of expressed milk.

Among the 65 women indicating having recovered at the time of reporting, the median duration of the reduced milk supply was 4 days (IQR = 4). Any burden was expressed by women in 189 reports (97.4%), with a mean level of burden of 3.9 (±1.0) on a 5-point scale.

Although it is clear that self-selected spontaneous reports cannot be used for the determination of incidence rates and causality, they should alert us to the possible degree of the magnitude of this adverse event. What is clearly needed to measure the true magnitude are data from a prospective monitored large population of vaccinated mothers.

A decreased milk supply was reported after vaccination with any type of available COVID-19 vaccine, but mainly after the Pfizer/BioNTech vaccine. However, this is not surprising as this is the most widely used vaccine in the Netherlands. In most cases, the event occurred within 1–2 days after vaccination. The age of the nursing infants indicates that decreased milk supply might happen at any time after birth, whereas the likelihood may be related to the occurrence of maternal systemic side effects. The observed duration of reduced milk production was longer than previous findings.^2,3 This might be due to reporting bias, as more severe cases are more likely to be spontaneously reported.

Although the likelihood of occurrence of reduced milk supply appears to be low and transient, it may well be a substantial burden for lactating women. Alerting mothers as to the degree and true nature of the risk of this adverse event may potentially reassure mothers and contribute to higher COVID-19 vaccination rates among hesitating pregnant women.