Are vaccines safe during pregnancy?

The verdict

Yes, and for several of them the question is not whether they are permitted but whether you have had them. These are among the few interventions in pregnancy that are actively recommended rather than merely tolerated.

The NHS states that some vaccines, such as the inactivated seasonal flu vaccine and the whooping cough vaccine, are recommended during pregnancy to protect the health of you and your baby. ACOG states that vaccines are safe for both of you, that vaccination is one of the most important things you can do to protect your health and your baby's, and that vaccines have been safely given to millions of pregnant women for more than 50 years.

The dividing line runs between inactivated and live vaccines, not between vaccines and no vaccines.

The four that are recommended

Flu. The NHS explains the reason plainly: during pregnancy your immune system is weakened to protect the pregnancy, which can mean you are less able to fight off infections such as flu. Pregnant women are more likely to get complications such as pneumonia and more likely to be admitted to hospital. ACOG advises a flu vaccine if you are pregnant during flu season, October through May in the US, taken as early in the season as possible. Both specify the injected inactivated vaccine; ACOG names the live attenuated nasal spray as one to avoid in pregnancy.

Whooping cough. The mechanism is antibody transfer. The NHS explains that when you have the vaccination in pregnancy your body produces antibodies which pass to your baby, giving high levels of protection until they can have their own vaccination at eight weeks old. Most babies who get whooping cough are admitted to hospital.

RSV. The NHS states that RSV can cause serious lung infections including pneumonia and bronchiolitis, and that vaccination in pregnancy passes protection to your baby, making severe RSV less likely for the first six months after birth.

COVID-19. ACOG states that if you are pregnant and not up to date on your COVID-19 vaccines you should get the currently recommended vaccine, and that this vaccine is safe during pregnancy. The NHS states that most people who are eligible for the COVID-19 vaccine can have it, including if you are pregnant or breastfeeding, and that in winter it can be given at the same time as the flu or RSV vaccine. UK eligibility is set by seasonal campaign rather than by pregnancy alone, so check what is being offered when you are pregnant.

Which vaccines are avoided

The NHS states that most vaccines using live bacteria or viruses are not recommended during pregnancy because of concerns they could harm the baby in the womb, and lists BCG, MMR, oral typhoid and yellow fever as live vaccines. ACOG's list of vaccines not to be given in pregnancy is the live attenuated nasal spray flu vaccine, MMR and chickenpox, plus HPV, which is not live but is still not given in pregnancy.

Two important qualifications sit alongside that. The NHS says there is no evidence that any live vaccine causes birth defects, and that a live vaccine may sometimes be used in pregnancy if the risk of infection is greater than the risk of vaccination. Yellow fever is its worked example: it is a live vaccine, but most people who get severe yellow fever die from it, so vaccination may be considered necessary for travel to areas where it is common.

The NHS also confirms that some vaccines, such as tetanus, are perfectly safe to have during pregnancy if needed, and that non-live vaccines are safe in pregnancy generally.

Where guidance genuinely differs: the timing

Both countries recommend the same vaccines. They schedule two of them differently, and the gaps are wide enough to matter if you move, travel or read the wrong country's advice.

Whooping cough: UK 20 weeks, US 27 to 36 weeks. The NHS says you usually have the whooping cough vaccine at 20 weeks pregnant, that you can have it from 16 weeks, and that to give your baby the best protection you should have it before 32 weeks, though you can still have it later if you miss that window. The CDC advises a dose of Tdap during each pregnancy irrespective of prior history, with optimal timing between 27 and 36 weeks, and notes that vaccinating earlier within that window maximises antibody transfer. ACOG gives the same 27 to 36 week range. A woman vaccinated on the NHS schedule at 20 weeks would be considered early by US guidance; a woman following US guidance at 34 weeks would be considered late by UK guidance.

RSV: UK from 28 weeks, US 32 to 36 weeks and seasonal. The NHS says you should be offered the RSV vaccine around the time of your 28-week antenatal appointment, that getting vaccinated as soon as possible from 28 weeks gives the best protection, and that it can be given later including up until you go into labour. The CDC recommends maternal Pfizer RSVpreF at 32 weeks and 0 days to 36 weeks and 6 days of gestation, administered between September and January for most of the continental United States, and specifies that only the Pfizer product may be given in pregnancy. It also states that if a woman has already had a maternal RSV vaccine in a previous pregnancy, another dose is not currently recommended, and the infant should receive nirsevimab instead. The UK offers RSV vaccination year-round from 28 weeks; the US ties it to a season.

Flu is the one they agree on, in substance if not in wording: inactivated vaccine, any trimester, as early in the season as possible.

What about the ingredients?

ACOG addresses the two questions people ask most. On mercury: there is a tiny amount of thimerosal in a small number of vaccines, it is safe and not harmful during pregnancy, and it uses ethylmercury, which is different from the mercury found in fish and leaves the body naturally. Most new vaccines come in single-dose vials and no longer contain it.

On side effects: some people have none, others have mild effects such as a sore arm or a low fever that resolve within a day or two, and severe reactions are rare. If you have ever had a reaction to a vaccine, that is a conversation to have before the appointment rather than a reason to skip it.

If you have missed one

Very little of this is time-barred. The NHS says that if you miss the whooping cough vaccine before 32 weeks you can still have it later, and that if you are 20 weeks pregnant and have not been offered it you should contact your midwife or GP surgery. It says the same for RSV from 28 weeks onwards. Flu vaccination is available for the whole season.

The CDC adds the postnatal fallback for whooping cough: for women not previously vaccinated with Tdap, if it is not given during pregnancy it should be administered immediately after birth. That protects you rather than transferring antibodies to your baby, which is why the antenatal dose is preferred.