Vaccination Q&A

Fact: Hygiene and nutrition helped reduce deaths but did NOT eliminate the germs. Measles, polio and pertussis only fell dramatically after vaccines reached the whole population. For example, when the UK and Japan cut back on pertussis vaccination in the 1970s over fears of side effects, huge pertussis epidemics returned almost immediately, with dozens of deaths.

Source: US CDC & Vietnam Association of Preventive Medicine

Fact: This is a probability paradox. In a population with very high vaccination coverage (for example 98%), vaccinated people vastly outnumber unvaccinated people. No vaccine is 100% effective (most are 85–95%). In a school of 1,000 pupils where 995 are vaccinated and 5 are not, when measles breaks out, 100% (5/5) of the unvaccinated pupils fall ill, while only 2% (20/995) of the vaccinated pupils get a mild illness. There are 20 cases among vaccinated pupils compared with 5 among unvaccinated pupils, but the vaccinated group is far better protected.

Source: 2026 vaccination recommendations

Fact: A newborn's immune system can respond to thousands of antigens at once. Food, air and the bacteria that normally live in the mouth and nose expose a child to hundreds of times more antigens every day than a vaccine does. Combination vaccines, or several vaccines given at the same visit, protect children as early as possible during their highest-risk period without increasing side effects.

Source: WHO & National Institute of Hygiene and Epidemiology

Fact: Any interruption of a vaccination schedule (except oral typhoid vaccine) does NOT require starting again. The body keeps its "immune memory". You only need to continue with the missing doses at the recommended intervals.

Source: 2026 lifelong vaccination schedule recommendations

The 4-day rule: a dose given 1 to 4 days before the minimum due date is still considered valid. A dose given 5 or more days early does not count and must be repeated once the required interval has passed.

Source: Ministry of Health of Vietnam

Invasive meningococcal disease (IMD) is a fulminant infection caused by the bacterium Neisseria meningitidis, including purulent meningitis and sepsis. It is extremely dangerous because it starts suddenly in completely healthy people; the first symptoms look like flu, but it can progress to endotoxic shock and death within 24 hours of the first symptom. The case fatality rate is 10–15%, and 20% of survivors are left with permanent disabilities.

Source: Book: Prevention of meningococcal disease in Vietnam (2024)

Serogroup W (W-135) has the highest case fatality rate (CFR) worldwide, up to 19–21.4% (according to data from Europe and the US CDC), followed by serogroup C (~16%), Y (~8%) and B (~8%). In Vietnam, serogroup B accounts for the majority of cases (>90%), but serogroups W and C have also been recorded causing invasive disease.

Source: Book: Prevention of meningococcal disease in Vietnam (2024)

YES. Meningococci have 12–13 different serogroups (6 main ones: A, B, C, W, X and Y). Immunity after infection with one serogroup does not protect against the others. In addition, natural antibodies tend to wane over time. People who have had the disease therefore still need to be vaccinated for full protection.

Source: US CDC & Vietnam Association of Preventive Medicine

VA-Mengoc-BC (Cuba) is made with outer membrane vesicle (OMV) technology; its main antigen is the PorA protein of a Cuban bacterial strain. Because PorA mutates very easily (>600 variants), OMV vaccines only give optimal protection against the specific strain and have limited effectiveness in children under 2 years (-37% in a study in Brazil).
In contrast, 4CMenB (Bexsero) uses reverse vaccinology: 2,158 genes were screened to select 3 highly conserved recombinant proteins (fHbp, NadA, NHBA), combined with PorA P1.4. This gives coverage of >80% of serogroup B meningococcal strains worldwide and strong protection from 2 months of age.

Source: Book: Prevention of meningococcal disease in Vietnam (2024)

YES, AND THEY SHOULD. People who have completed VA-Mengoc-BC should also receive 4CMenB (Bexsero) to broaden coverage against the many serogroup B strains. Because both are inactivated vaccines with different antigens, Bexsero can be given at any time after the last VA-Mengoc-BC dose, with NO INTERVAL NEEDED.

Source: Vietnam Association of Preventive Medicine

According to the expert consensus at the workshop on introducing serogroup B meningococcal vaccination (19/10/2023), preventive paracetamol before Bexsero is NOT recommended. Parents should only give paracetamol for fever when the child's temperature is 38.5°C or higher.

Source: Vietnam Association of Preventive Medicine expert workshop, 2023

IT MUST NOT BE DELAYED. Premature babies have immature immune systems and do not receive enough antibodies from their mother during the last 3 months of pregnancy; their risk of serogroup B meningococcal disease is 25 times higher than that of other babies. They need to be vaccinated on schedule from 2 months of age. Extremely preterm babies (≤ 28 weeks) need their breathing monitored for the first 48–72 hours at a health facility.

Source: ECDC & the 2024 meningococcal prevention book

YES, THEY ARE INTERCHANGEABLE. Medical data from 16 randomised trials in 20,000 people confirm that the MenACWY conjugate vaccines have equivalent safety and protection, and they can replace each other for later doses if the vaccination clinic does not have the vaccine used for the first dose.

Source: CDC Pink Book & ACIP

NO. Although Menactra uses diphtheria toxoid (DT) as the carrier protein in the conjugate to engage immune memory T cells, this amount only acts as a carrier; there is no scientific evidence that it protects people against diphtheria. Children still need a separate diphtheria-containing vaccine (such as 6-in-1, Tetraxim or Tdap).

Source: Menactra prescribing information & European Pharmacopoeia

Close contacts (household members, people sharing an office, classmates, anyone in direct contact with nose and throat secretions during the 7 days before onset) should take preventive antibiotics within 24 hours of the case being identified:
- Ciprofloxacin: a single oral dose of 500 mg for adults and children over 12 years (contraindicated in pregnant and breastfeeding women).
- Rifampicin: adults 600 mg twice a day for 2 days; children 1–12 years 10 mg/kg twice a day for 2 days; infants under 12 months 5 mg/kg twice a day for 2 days.
- Azithromycin: a single dose of 500 mg for adults (safe in pregnancy and breastfeeding); children 10 mg/kg as a single dose.

Source: Ministry of Health Decision 3897/QĐ-BYT

ABSOLUTELY NOT. This is a false rumour spread on social media. The WHO Global Advisory Committee on Vaccine Safety (GACVS) and the US Centers for Disease Control and Prevention (CDC) have carried out very large cohort studies following millions of women after HPV vaccination for nearly 20 years. The results confirm that the HPV vaccine does not cause primary ovarian insufficiency, does not reduce fertility and does not cause infertility. On the contrary, vaccination prevents precancerous lesions that would otherwise need treatment by cervical conisation (LEEP), a procedure that can later lead to cervical insufficiency, miscarriage or premature birth.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

YES, THEY SHOULD. Even after becoming sexually active or being infected with one or more HPV types, very few people are infected with all 9 types covered by Gardasil 9 at the same time. Vaccination protects against the other high-risk HPV types they have not been exposed to, and also prevents reinfection with the same type after the body has cleared it, because natural immunity after HPV infection is weak and does not produce lasting protective antibodies.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

NOT AT ALL. All reputable health organisations (WHO, US CDC, Vietnam Association of Preventive Medicine) recommend that no HPV DNA test or gynaecological examination is needed before HPV vaccination. Screening is for the early detection of precancerous lesions at regular intervals; it is not a condition for deciding whether someone can be vaccinated.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

Men need the HPV vaccine for 4 key medical reasons:
1. HPV infection is as common in men as in women, but men clear the virus more slowly and produce fewer natural antibodies than women.
2. There is NO regular screening for men (like the Pap smear for women); HPV-related lesions in men are usually only found once they have become lumps or cancer.
3. HPV causes 90% of genital warts, 60% of penile cancers, 90% of anal cancers and more than 70% of oropharyngeal cancers in men.
4. Vaccinating men stops them from passing the virus on to their sexual partners in the community.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

The minimum interval rules are strict:
- 2-dose schedule (9–14 years): dose 2 must be at least 5 MONTHS (150 days) after dose 1. If dose 2 is given less than 5 months after dose 1 (for example after only 2 or 3 months), it does NOT COUNT towards completing the 2-dose schedule, and a THIRD DOSE is required (at least 3 months after dose 2).
- 3-dose schedule (15–45 years): the minimum interval between doses 1 and 2 is 1 MONTH (4 weeks); between doses 2 and 3 it is 3 MONTHS (12 weeks); and dose 3 must be at least 5 MONTHS (150 days) after dose 1. A dose given before the minimum interval is considered invalid and must be repeated.

Source: Gardasil 9 prescribing information & the HPV Q&A book

THERE IS NO NEED TO START AGAIN. The immune system keeps memory B lymphocytes. However long the interruption (even 5 or 10 years), the person only needs to receive the missing doses to complete the schedule; the doses already given remain valid.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

THERE IS NO INDICATION AT ALL FOR TERMINATING THE PREGNANCY. The HPV vaccine is a non-live recombinant vaccine (it contains only empty L1 protein shells and no viral genetic material, RNA or DNA). Global pregnancy surveillance data show that the rates of miscarriage and birth defects in women vaccinated by mistake during pregnancy are no different from the natural rates in the community. What to do: postpone the remaining doses, continue normal antenatal care, and complete the remaining doses after the baby is born.

Source: US CDC & Vietnam Association of Preventive Medicine

YES. People who have had all 3 doses of quadrivalent Gardasil (types 6, 11, 16, 18) and want to extend protection to 5 additional high-risk types (31, 33, 45, 52, 58) can receive the full 3-dose Gardasil 9 schedule at 0, 2 and 6 months. The recommended interval is at least 12 months after the last Gardasil 4 dose.

Source: Gardasil 9 prescribing information & the HPV Q&A book

IT IS STRONGLY RECOMMENDED. Clinical studies show that women who receive the HPV vaccine after conisation for precancerous lesions (CIN 2, CIN 3) have a 60–80% lower risk of recurrent lesions than those who are not vaccinated. The best time to vaccinate is just before or just after the procedure.

Source: Book: Questions and answers on HPV vaccines (Vietnam Association of Preventive Medicine & MSD, 2024)

The key difference lies in the technology and the risk of ADE (antibody-dependent enhancement):
- Dengvaxia (Sanofi): uses a yellow fever 17D virus backbone. In people who have never had dengue (seronegative), the vaccine acts like a first natural infection. When such a person is later bitten by a mosquito and naturally infected with dengue, non-neutralising antibodies trigger ADE, causing the virus to multiply massively, which leads to severe dengue and more hospital admissions. WHO therefore requires a blood test to confirm previous dengue infection before Dengvaxia can be given.
- Qdenga (Takeda): uses a dengue type 2 virus backbone (strain PDK-53) and stimulates both neutralising antibodies and broad T cell-mediated immunity. A phase 3 clinical trial with 4.5 years of follow-up confirmed that Qdenga is safe and highly effective in BOTH PEOPLE WHO HAVE AND HAVE NOT HAD DENGUE, so NO screening test is needed before vaccination.

Source: Book: Dengue fever (Vietnam Association of Preventive Medicine, 2024)

According to professional guidance: Qdenga is a live attenuated vaccine. EXCEPT for yellow fever vaccine, Qdenga must NEVER BE GIVEN ON THE SAME DAY as other live attenuated vaccines (such as measles, mumps, rubella or chickenpox). The required interval between Qdenga and other live vaccines is at least 28 days (4 weeks), so that they do not reduce each other's immune response.

Source: Qdenga prescribing information & the 2024 dengue book

CONTRAINDICATED. Unlike many other vaccines, the Qdenga dengue vaccine is clearly labelled by the manufacturer Takeda and the Ministry of Health as CONTRAINDICATED in breastfeeding and pregnant women. Women of childbearing age should use effective contraception for at least 1 month after each Qdenga dose.

Source: Qdenga prescribing information approved by the Ministry of Health

ADE (antibody-dependent enhancement) is the mechanism that explains why a second dengue infection is often more severe than the first. After recovering from one dengue type (for example DENV-1), the body has lifelong protective antibodies against DENV-1, but cross-reactive antibodies against the other types (DENV-2, 3, 4) fade after a few months. When a later mosquito bite causes infection with DENV-2, the old cross-reactive antibodies are not strong enough to neutralise the virus; instead they form complexes that bind to Fc gamma receptors on the surface of macrophages. This helps the new virus enter immune cells faster and multiply in huge numbers, causing a cytokine storm, increased capillary permeability and massive plasma leakage, which leads to shock and internal bleeding.

Source: Book: Dengue fever (Vietnam Association of Preventive Medicine, 2024)

In dengue, the platelet count falls sharply and the blood vessel walls are damaged, so the risk of spontaneous bleeding is very high. Aspirin and non-steroidal anti-inflammatory drugs (NSAIDs such as ibuprofen and diclofenac) inhibit platelet aggregation and irritate and ulcerate the stomach lining. Using them makes bleeding worse and can easily lead to massive gastrointestinal bleeding (vomiting blood, black stools), stomach perforation and death from blood loss. THE ONLY FEVER MEDICINE ALLOWED IS PARACETAMOL, at 10–15 mg/kg per dose (4–6 hours apart, no more than 60 mg/kg per day).

Source: Decision 2760/QĐ-BYT & the 2024 dengue book

According to Ministry of Health guidance, a person with dengue must be taken to hospital immediately if any of the following warning signs appear (usually between day 3 and day 7 of the illness):
1. Severe abdominal pain, with increasing and continuous pain over the liver (below the right ribs).
2. Persistent vomiting (3 times in 1 hour or 4 times in 6 hours).
3. Mucosal bleeding: bleeding gums, nosebleeds that are hard to stop, vomiting blood, black stools, blood in the urine, abnormal vaginal bleeding.
4. Fluid around the lungs or in the abdomen (swollen abdomen, laboured breathing).
5. Lethargy, drowsiness, restlessness, irritability, or cold hands and feet with sweating.
6. Tests: a high haematocrit (Hct) together with a rapid fall in platelets to below 100,000/mm3.

Source: Ministry of Health Decision 2760/QĐ-BYT