Dr. Vaccines
Types of Viruses and Types of Vaccines
Why some vaccines are one dose and others are five, why a few cannot be given in pregnancy, and why flu needs repeating every year.
Almost every practical question about a vaccine, how many doses, whether it is safe in pregnancy, whether it needs repeating, whether an immunocompromised person can have it, is answered by knowing what type of vaccine it is. The categories are not academic; they determine the rules.
Viruses themselves also vary in ways that shape whether a vaccine is even possible. Stable viruses that do not mutate much, like measles, produce vaccines that work for life. Rapidly mutating ones like influenza need annual reformulation, and some, like HIV and hepatitis C, have defeated vaccine development entirely.
At a glance
The main vaccine platforms and what each implies in practice.
| Vaccine or test | Status | Schedule | Detail |
|---|---|---|---|
| Live attenuated | Routine | Usually 1-2 doses, often lifelong protection | A weakened form of the real organism that replicates without causing disease. Produces strong, long-lasting immunity from few doses, but cannot be given in pregnancy or significant immunosuppression. Examples: MMR, varicella, oral polio, yellow fever, BCG. |
| Inactivated (killed) | Routine | Multiple doses, often with boosters | The organism is killed so it cannot replicate. Safe in pregnancy and immunosuppression, but immunity is weaker per dose, so more doses and periodic boosters are needed. Examples: inactivated polio, hepatitis A, rabies, injectable influenza. |
| Subunit / recombinant | Routine | Typically 2-3 doses | Contains only selected proteins rather than the whole organism, which reduces side effects. Often paired with an adjuvant to strengthen the response. Examples: hepatitis B, HPV, the recombinant shingles vaccine. |
| Toxoid | Routine | Primary course plus boosters every 10 years | Targets the toxin rather than the bacterium, for diseases where the toxin causes the harm. Immunity wanes, hence the 10-year boosters. Examples: tetanus, diphtheria. |
| Conjugate | Routine | Varies; effective from infancy | A bacterial sugar coat linked to a carrier protein. This is what makes these vaccines work in infants, whose immune systems respond poorly to sugars alone. Examples: Hib, pneumococcal conjugate, meningococcal ACWY, typhoid conjugate. |
| mRNA | Routine | Usually 2 doses plus boosters | Delivers genetic instructions for a single viral protein; your cells make it and the immune system responds. The mRNA does not enter the cell nucleus and degrades within days. Examples: several COVID-19 vaccines. |
| Viral vector | Routine | One or two doses | Uses a harmless modified virus to carry the genetic instructions. Examples: some COVID-19 vaccines, the Ebola vaccine. |
Why live vaccines have restrictions
A live attenuated vaccine contains an organism weakened enough that a healthy immune system controls it easily while building strong immunity. That is precisely why it is a problem when the immune system is not working normally: in significant immunosuppression, even a weakened organism can replicate more than intended.
The same logic explains the pregnancy precaution. The theoretical risk to a developing fetus has not translated into observed harm in follow-up studies, but the precaution stands because the alternative, waiting until after delivery, is almost always available.
Why influenza needs repeating and measles does not
Measles virus is genetically stable. The strain circulating today is close enough to the one in the vaccine that immunity acquired in childhood still recognises it decades later.
Influenza is the opposite. Its surface proteins accumulate mutations continuously, antigenic drift, and occasionally reassort into something substantially new. Last year's antibodies may not recognise this year's virus, which is why the vaccine is reformulated annually rather than boosted.
DNA and RNA viruses behave differently
RNA viruses: influenza, HIV, hepatitis C, coronaviruses, copy their genomes with enzymes that lack proofreading, so they mutate rapidly. That is why they are hard vaccine targets and why some have resisted vaccination entirely.
DNA viruses: hepatitis B, varicella zoster, HPV, herpes viruses, copy more accurately and mutate slowly, which makes them more tractable. Many also establish latency, persisting in the body for life. That is why chickenpox can return as shingles decades later, and why hepatitis B can sit silently before causing cirrhosis.
What adjuvants do
Purified subunit vaccines are safe but can be poorly immunogenic on their own. An adjuvant is added to provoke a stronger local immune response, effectively telling the immune system that the protein is worth taking seriously.
This is why some vaccines have noticeably more reactogenicity than others. The sore arm and day of tiredness after the recombinant shingles vaccine is the adjuvant working. It is why that vaccine remains effective in older adults, whose immune systems would otherwise respond weakly.
Frequently asked questions
What is the difference between live and inactivated vaccines?
Live attenuated vaccines contain a weakened but replicating organism, giving strong long-lasting immunity from few doses, but they cannot be given in pregnancy or significant immunosuppression. Inactivated vaccines contain killed organisms, are safe in those situations, but need more doses and periodic boosters.
Why does the flu vaccine need repeating every year?
Influenza mutates continuously, so its surface proteins change from season to season and last year's antibodies may not recognise this year's virus. The vaccine is reformulated annually to match the strains expected to circulate.
Are mRNA vaccines safe? Do they change my DNA?
No, they do not alter DNA. The mRNA never enters the cell nucleus, where DNA is kept, and it degrades within days. It simply provides temporary instructions for making one viral protein that the immune system then learns to recognise.
Why can infants have some vaccines but not others?
Infant immune systems respond poorly to plain bacterial sugar antigens. Conjugate vaccines solve this by linking the sugar to a carrier protein, which is why Hib, pneumococcal and typhoid conjugate vaccines work from a few months of age where the older polysaccharide versions did not.
Why is there no vaccine for HIV or hepatitis C?
Both are RNA viruses that mutate extremely rapidly and exist as many genetically distinct variants, so an immune response raised against one version may not recognise another. Both have resisted decades of vaccine development, though hepatitis C is now curable with antiviral treatment.
Not sure what you need?
Bring whatever records you have and our doctors will translate them into a plan. Where records are missing, serology can usually answer the question directly.
Before you rely on this page
This page is general health information, not personal medical advice. Vaccine schedules depend on your age, medical history, pregnancy status, medications and previous immunisations. Our doctors will confirm what is right for you at your appointment. National recommendations are also revised periodically. We review these pages against current Indian and WHO guidance.
Last reviewed: 2026-09-01
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