Parents often ask: how many injections can be given at one visit, how far apart should two vaccines be, and does a late schedule have to start again? Part VI of the book Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026) answers these questions.

Doses given earlier than scheduled

The interval between doses must not be shorter than the minimum interval, and no dose should be given before the minimum age.
A dose given up to 4 days before the minimum age or interval is still counted as valid.
A dose given 5 or more days early is not valid. It must be repeated, after the required interval from the early dose.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 2.1, p. 199.

Late doses

A longer interval than recommended does not reduce the antibody level once all doses have been given. However, the body is not fully protected until the series is complete. Except for oral typhoid vaccine, an interrupted schedule does not have to be restarted and no extra doses are needed. Simply give the missing doses as soon as possible.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 2.1, pp. 199–200.

Several vaccines at one visit

Giving all age-appropriate vaccines at the same visit is recommended when the child has no contraindications. The immune response and side effects are the same as when each vaccine is given separately.
Inactivated vaccines can be given at the same time as, or before or after, any other vaccine, with no interval needed.
Live vaccines can be given together on the same day. If they are not given on the same day, they must be at least 4 weeks (28 days) apart.
Exception: the Qdenga dengue vaccine is not given on the same day as other live vaccines; they must be 28 days apart.
Some same-day combinations have supporting data: MMR and chickenpox; pneumococcal and inactivated influenza; Tdap and inactivated influenza.
There is no exact limit on the number of injections at one visit, but the first vaccinations in life should not include too many injections at once.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, sections 2.1–2.2, p. 200.

Combination vaccines

Combination vaccines (5-in-1, 6-in-1…) reduce the number of injections. Children are more likely to be vaccinated completely and on time, and families make fewer trips.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 2.3, p. 201.

After immunoglobulin or a blood transfusion

Blood and antibody-containing products can reduce the response to measles and rubella vaccines for about 3 months. These live vaccines therefore have to be postponed. Yellow fever, live influenza, rotavirus and oral typhoid vaccines are not affected.

If MMR or chickenpox vaccine was given and an antibody-containing product was then given within 14 days, the vaccine needs to be repeated. Inactivated vaccines are hardly affected and can be given at the same time as antibody products, at a different site.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 2.4, p. 201.

Switching vaccine brands part-way through

In principle, combination vaccines from different manufacturers should not be interchanged.
If the same brand is not available, or it is not known which brand was used before, give the vaccine that is available. Do not delay vaccination just because the same brand is missing.
Hib conjugate, hepatitis B, hepatitis A, rotavirus and quadrivalent meningococcal conjugate vaccines from different manufacturers can be interchanged.
Children who need 2 doses of influenza vaccine in their first year should receive the same type.
For meningococcal group B vaccines, the meningococcal book advises completing the course with the same vaccine.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 2.5, pp. 201–202; Prevention of meningococcal disease in Vietnam (Vietnam Association of Preventive Medicine), Lesson 8, p. 248.

Can you be vaccinated while taking medicines?

Antibiotics are not a contraindication to vaccination. The exception is oral typhoid vaccine.
Antiviral medicines for herpes reduce the effectiveness of live chickenpox and shingles vaccines. They must be stopped at least 24 hours before vaccination.

Source: Recommended vaccination schedules for all ages in Vietnam (Vietnam Association of Preventive Medicine, 3rd edition, 2026), Part VI, section 6.1, pp. 204–205.