Key messages
Vitamin D supplements compared with placebo (a sham treatment) may slightly reduce the number of young children who need a doctor or hospital visit for sudden infections in the lungs and airways (acute respiratory infections (ARIs)). They probably do not change how often each child visits a doctor or hospital. Higher doses of vitamin D compared with lower doses probably do not reduce how many children need a doctor or hospital visit for an ARI and do not reduce how often each child visits.

Vitamin D supplements appear safe, showing little to no impact on the risk of developing high blood calcium levels.

High-quality studies comparing vitamin D supplements with placebo are needed to find out whether vitamin D prevents ARIs in young children.

What are acute respiratory infections?
Acute respiratory infections are sudden infections that can affect the nose, ears, throat, airways, or lungs and are a major cause of illness and death in children younger than five years of age. Typically caused by viruses or bacteria, ARIs are a common reason for doctor or hospital visits in this age group. Symptoms often include a cough, runny nose, sore throat, fever, or difficulty breathing. In children under five years, symptoms may also include fast breathing, wheezing, poor feeding, vomiting, or being unusually sleepy or irritable.

What is vitamin D, and why is it important?
Vitamin D is a nutrient best known for supporting bone health. It also plays a role in developing a healthy immune system (the body's defence against infections). People get vitamin D from sunlight, a small number of foods, foods with vitamin D added, or from supplements. Many pregnant women and young children around the world do not get enough vitamin D because of limited sun exposure, diet, or other factors, which may increase the risk of ARIs in children.

What did we want to find out?
This review aimed to find out whether vitamin D supplementation during pregnancy or early childhood:

reduces the number of young children who require a doctor or hospital visit for an ARI;

reduces how often each child needs to visit a doctor or hospital for ARIs;

causes harm by increasing calcium levels in the blood (hypercalcaemia) of pregnant women or children.

What did we do?
We searched for studies that compared either:

giving vitamin D versus a placebo (a sham treatment); or

using a higher dose of vitamin D compared to a lower dose.

We compared and summarised the findings and assessed how confident we could be in the evidence, based on the quality of the study methods.

What did we find?
We found 107 studies involving 31,521 pregnant women and children. Vitamin D was given during pregnancy, early childhood, or both. The amount and timing of vitamin D varied. It was most often given daily, but sometimes less frequently, including as large single doses. The studies were conducted in many countries. For each participant, the study duration ranged from one day to 18 months. The studies were conducted in hospitals, day-care centres, communities, or homes.

When vitamin D is compared with a placebo, supplementation may slightly reduce the number of children who need a doctor or hospital visit for an ARI. However, vitamin D does not reduce how often each child visits a doctor or hospital for an ARI.

When higher doses of vitamin D are compared with lower doses, higher doses probably do not reduce the number of children who need a doctor or hospital visit for an ARI and do not reduce how often each child visits.

Vitamin D supplementation appears to be safe. High blood calcium levels were rare, and vitamin D supplementation may have little to no effect on the risk of developing high blood calcium levels in pregnant women and young children.

What are the limitations of the evidence?
We have low confidence in the evidence about whether vitamin D reduces doctor or hospital visits for ARIs. Results varied between studies, with some suggesting a small benefit and others showing no clear effect.

Many studies were small, which limits how reliable they are. Additionally, differences in vitamin D dosing and how ARIs were measured made it difficult to combine and interpret the results across studies.

How up to date is this evidence?
This evidence is current to 18 March 2025.