Vaccines are safe, but not completely free of risk. Most reactions after vaccination are mild and go away by themselves. Serious reactions are very rare, but they need to be spotted early. This article summarises the guidance in Part V of the book Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026).

Where reactions after vaccination come from

The book uses the term "adverse event following immunization": any health event that happens after vaccination, which is not necessarily caused by the vaccine. According to the WHO classification, the cause may be:

A reaction to the vaccine itself.
An error in storage, prescription or injection technique.
An anxiety-related reaction to the injection.
Another illness that happens to occur at the same time.
Unknown.

According to the book, fever is the most common general reaction. Redness and swelling at the injection site are usually mild. Anaphylaxis is very rare. Fainting after vaccination is most common in young people.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, section 1, pp. 189–190.

The first 30 minutes at the vaccination site

The vaccinated person must stay at the vaccination site for 30 minutes so that health workers can observe them. Parents should also watch their child.

A child who is well:

Pink skin and lips, no rash.
Alert, not restless and not drowsy.
Not breathing fast, no difficulty breathing; temperature below 37.5°C.
Redness and swelling at the injection site smaller than 5 cm.

Tell a health worker immediately if you notice any of these signs:

The child is agitated, panicked or crying for a long time.
Hives, itching, swollen lips or face.
Difficulty breathing, chest tightness, noisy breathing.
Abdominal pain, vomiting, loss of bladder or bowel control.
Cold hands and feet, fainting.
Seizures.

Circular 13/2026/TT-BYT (in force from 01/7/2026) requires vaccination facilities to observe vaccinated people for at least 30 minutes and to instruct families to monitor them at home for at least 24 hours.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, section 3, pp. 192–193; Circular 13/2026/TT-BYT, Article 12(2).

The first 24 hours at home

Monitor at home for at least 24 hours. For children, the person watching must be an adult who knows how to care for children. Look out for:

General condition, alertness, eating and sleeping.
Breathing rate.
Temperature and any rash.
Redness or swelling at the injection site.

Breastfeed or give the child more fluids than usual. Hold and watch the child often, and avoid touching or pressing on the injection site.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, section 4.1, p. 193.

Managing common reactions

Mild fever below 38.5°C: drink plenty of fluids, eat normally, rest in a well-ventilated place. Children with heart disease, pneumonia or a history of febrile seizures may be given fever medicine above 38°C, as advised by a health worker.
High fever above 38.5°C: give paracetamol, plenty of fluids, and sponge with lukewarm water. If the fever does not come down after 1–2 hours, ibuprofen can be added if there is no contraindication.
Swelling, redness and pain at the injection site: usually go away within a few days to a week. Do not put anything on the injection site, as it may cause infection.
After BCG, a lymph node may appear in the armpit on the same side for 2–6 months. It usually heals by itself. If the node sticks to the skin or discharges fluid, go to a health facility.

Note: the advice above is only for reactions after vaccination. People who may have dengue must not take ibuprofen or aspirin; see the article on dengue.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, sections 5–6, pp. 194–196.

When to go to hospital immediately

Go to a health facility immediately if there is any of these signs:

Fever above 39°C.
Seizures, or exhaustion and no response when called.
Blue colour, difficulty breathing.
Fussiness or high-pitched crying for more than 3 hours.
Poor feeding, a rash, or ordinary reactions that last more than 1 day.

Anaphylaxis usually happens during or right after the injection, with signs such as agitation, itchy rash, hives, swelling; a fast weak pulse and low blood pressure; difficulty breathing or choking; cramping abdominal pain; dizziness and seizures. This is why people must stay for 30 minutes of observation. Every vaccination facility must be ready to treat anaphylaxis according to the Ministry of Health protocol.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, sections 4.2 and 6.2, pp. 193–196.

Before the next vaccination

Tell the screening doctor if the child had a high fever, prolonged crying, a rash or widespread swelling at the injection site after the last dose. People whose reactions get stronger with each dose of the same vaccine need screening and vaccination at a hospital.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part V, sections 2.1 and 2.2.4, pp. 190–192.