A vaccine works by exposing the immune system to a weakened or inactivated piece of a disease-causing organism, or to a small protein from it, so the body learns to recognise the real pathogen and mount a fast targeted response if it ever encounters it. The mild side effects that follow — low-grade fever, fussiness, sleepiness, redness or swelling at the injection site, and a brief loss of appetite — are the visible outcome of this immune learning process. In simple terms, the small reaction is the vaccine doing its job.

Most reactions are local (at the injection site) or mild systemic (low fever fussiness sleepiness), begin within a few hours of the shot, peak in the first 24 hours, and resolve in one to three days without treatment beyond comfort. A subset of vaccines have predictable patterns — BCG develops a scab and scar over weeks, MMR can cause a brief fever and rash five to twelve days later, and DTwP is known to cause higher fever than DTaP — and recognising these helps the parent know what is expected. The pediatrician will brief the family at every vaccination visit.

Serious reactions are extremely rare. Anaphylaxis (a severe allergic reaction) occurs in less than one in a million doses and the observation period of fifteen to thirty minutes after the shot at the clinic is specifically to catch it early, when it is fully treatable with epinephrine. The clear framing for the family is that mild reactions are normal and reassuring, that the vast majority of babies have no problem at all, and that the safety record of routine childhood immunisation is one of the strongest in modern medicine.

The most common side effects after almost any vaccine in the first year are low-grade fever (under 101 F or 38.3 C), mild redness or swelling at the injection site (usually less than five centimetres across), fussiness or irritability for 24 to 48 hours, increased sleepiness or sometimes a single restless night, and a brief mild loss of appetite with smaller feeds. These are all part of the normal immune response and almost always resolve completely within one to three days without any intervention beyond gentle care and reassurance. Many babies have no symptoms at all.

The local injection-site reaction is usually limited to a small area of redness warmth and mild firmness under the skin; a slight pea-sized lump that persists for a week or two is also common and not a problem. Fever of 100 to 100.9 F (37.8 to 38.3 C) in the first 24 to 48 hours after a shot is the body's normal inflammatory response and does not by itself need treatment — the baby may be a little warmer and may want extra cuddles, all of which are managed with skin-to-skin light clothing extra breastfeeding and patience.

The pattern that should reassure rather than alarm the family is symptoms that begin within 6 to 24 hours of the vaccine, peak in the first day, and steadily improve over the next two days. Symptoms that begin three or more days after a vaccine are often unrelated to the vaccine and may be a coincidental infection that the baby would have caught anyway — in that case the same illness-evaluation rules as for any baby fever apply, and a pediatrician review is sensible if the baby is unwell. For broader fever evaluation see Baby Fever in Indian Infants: When to Worry, Paracetamol Dosing, and ER Signs.

The BCG vaccine given at birth to protect against tuberculosis has a unique and very predictable local reaction that worries many first-time parents because it looks dramatic but is entirely normal. Within two to six weeks of the injection (on the upper left arm) a small red papule appears, then enlarges to a small bump, develops a soft scab or shallow ulcer at four to twelve weeks, and finally heals over several months to leave a small round scar visible for life. The whole sequence is normal and expected.

Do not apply anything to the BCG site. Do not put antiseptic creams, turmeric, neem paste, breast milk, ash, or any home or pharmacy remedy on it — none of these helps and several can cause secondary infection. The site is allowed to ooze a little clear fluid and form its own scab; cover it loosely with a clean dry gauze if the discharge is staining clothes, change the gauze daily, and keep the area clean with plain water during the daily bath without scrubbing. The scar by six months is the marker that the vaccine has taken.

The reaction is not a sign of infection and does not need antibiotics. The few situations that do need pediatrician review are a large abscess that is rapidly growing and very painful, marked swelling of the lymph node in the armpit on the same side (a small firm node is normal, a swollen tender mass needs review), or a deep ulcer that is not healing by twelve weeks. For more detail on the BCG site progression see baby-bcg-reactions.

The DTwP (whole-cell pertussis) vaccine, given as part of the pentavalent shot at 6, 10 and 14 weeks in India's free UIP, is the most reactive of the standard childhood vaccines and produces the highest fever and most local swelling. A fever up to 102 F (39 C), a firm tender red lump at the site for two to three days, marked fussiness and prolonged crying (over three hours of inconsolable crying in about 1 to 3 percent of babies), and a restless night are all expected after a DTwP dose, peaking in 24 hours and settling by 72 hours.

IAP guidance for families who have had a severe reaction to DTwP — high fever above 104 F, prolonged crying over three hours, a hypotonic episode (the baby becoming floppy briefly), or a febrile seizure — is to switch to DTaP (acellular pertussis) for the remaining doses. DTaP is available in India as Pentaxim, Hexaxim, Easy-Six and Infanrix-Hexa, costs around 2000 to 3500 rupees per dose privately, and has lower rates of fever and local reaction than DTwP. The free UIP pentavalent is excellent value; DTaP is the upgrade option.

The right approach for every DTwP or pentavalent dose is to plan for paracetamol after the shot, light clothing for the evening, extra breastfeeding on demand, and a calm low-stimulation environment for the rest of the day. The baby will usually be back to normal by the third day and the protection against pertussis (whooping cough) is one of the most important interventions of the entire first year.

The MMR vaccine (measles mumps rubella) given at 9 months in India and again at 16 to 18 months has a different reaction pattern because it contains a weakened live virus that takes a week or so to multiply enough to trigger a small immune response. About five to twelve days after the shot, around five to ten percent of babies develop a low-grade fever (under 102 F) and a faint pink rash lasting a day or two. This is not measles, is not contagious, and needs no treatment beyond comfort and paracetamol if the baby is uncomfortable.

A small number of babies have a febrile seizure (a brief convulsion triggered by a sudden rise in temperature) in this same five-to-twelve-day window; this is frightening for the family but is generally not harmful, does not cause epilepsy, and is also a feature of high fevers from any cause. If a baby has a seizure of any kind after a vaccine the family should call the pediatrician the same day for an in-person review and for advice on future doses. The pediatrician will usually still recommend the MMR booster on schedule.

The persistent myth that MMR causes autism originated from a single 1998 paper by Andrew Wakefield that was later found to be fraudulent, retracted by the journal, and the author was struck off the UK medical register. Every subsequent large study (involving millions of children across many countries) has shown no link between MMR and autism. The IAP, the WHO, and every reputable pediatric body strongly recommend MMR on schedule. Skipping MMR exposes the baby to genuine measles, which is one of the most dangerous childhood infections in India.

Post-vaccine fever in the first 24 to 48 hours is the body's normal inflammatory response and is managed with physical comfort and, if the baby is uncomfortable, paracetamol. Start with the basics: undress the baby to a single light cotton layer or a nappy alone, keep the room at a comfortable 22 to 24 C (use a fan or AC rather than wrapping the baby in extra clothing), offer extra breastfeeds or formula on demand, and do skin-to-skin with the parent because the parent's body temperature regulates the baby's.

Paracetamol (Calpol drops or Crocin drops in India, around 40 to 80 rupees per bottle) is the only fever medicine considered safe in the first year of life and is given at 15 mg per kg of body weight every six hours as needed. For a typical 5 kg baby this is 75 mg per dose, which is 1.875 ml of the 100 mg per 1.5 ml drops; for a 7 kg baby it is around 105 mg. Use the marked dropper from the same bottle and confirm the dose with the pediatrician at the vaccination visit.

Tepid sponging (a soft cloth wrung out in lukewarm water gently wiped over the forehead arms and legs) is useful for very high fever while the paracetamol takes effect. Do not use cold water or ice — these cause shivering which paradoxically raises the temperature. Do not give aspirin to a baby (it can cause Reye's syndrome). Do not give ibuprofen under six months without specific pediatrician advice. Continue breastfeeding on demand; offer cooled boiled water in sips for older babies on solids.

While most post-vaccine reactions are mild and self-limiting, a small set of warning signs need a same-day call or visit to the pediatrician. Call for fever above 103 F (39.4 C) that does not come down with paracetamol, any fever in a baby under three months, fever that persists more than 48 hours after a vaccine (most vaccine fever resolves by 48 hours; beyond that it is more likely an unrelated infection), or fever associated with the baby looking visibly unwell rather than just a little warm.

Other red flags are persistent inconsolable crying for more than three hours (the high-pitched cri-encephalique pattern that the IAP specifically asks parents to report), a baby who is unusually lethargic and difficult to wake or who is not responding normally to the family, any kind of convulsion or seizure (a brief stiffening or jerking of the limbs with rolling of the eyes), persistent vomiting that prevents keeping feeds down, swelling at the injection site larger than 5 cm or extending up or down the limb, and any sign of breathing difficulty fast breathing chest retractions or noisy breathing.

Call an ambulance or go to the nearest emergency for any of the anaphylaxis warning signs (covered in the next section), a baby who is unresponsive or floppy, a baby who is turning blue around the lips or face, or a febrile seizure that lasts more than five minutes or that is followed by the baby not recovering normally. The Apollo and 1mg telemedicine services (consult fee around 500 to 2000 rupees) and the eSanjeevani public telemedicine service (free) are useful first contacts when the family is unsure whether to go to the hospital.

Anaphylaxis is a severe whole-body allergic reaction that occurs in less than one in a million vaccine doses and is the rarest but most serious of recognised vaccine reactions. It happens within minutes to half an hour of the injection (which is why every clinic asks the family to stay for 15 to 30 minutes of observation after the shot), is fully treatable with intramuscular epinephrine kept ready in every clinic, and has an excellent outcome when recognised early. The observation period exists so staff catch any anaphylaxis at the clinic rather than the family identifying it at home.

The signs of anaphylaxis are a sudden generalised hives or rash (raised red itchy bumps spreading across the body), swelling of the face lips eyelids or throat, difficulty breathing or noisy stridor, a sudden change in colour to pale or bluish, sudden floppiness or loss of consciousness, and sometimes vomiting and watery diarrhoea. Any of these in the first half hour after a vaccine is a medical emergency and is treated with intramuscular epinephrine, oxygen, antihistamines, and steroids by the clinic staff or the emergency hospital, and the baby is admitted for monitoring overnight.

After a confirmed anaphylactic reaction to a vaccine the pediatrician will refer the family to a pediatric allergist for evaluation and for advice on future vaccines — most babies can still receive most vaccines, sometimes with a substituted preparation or with an extended observation period in the hospital. The clear framing for the family is that anaphylaxis is genuinely very rare, that the observation period at the clinic is designed to catch it early, and that the protection from preventable diseases is overwhelmingly worth the very small risk.

India follows two overlapping schedules: the free Universal Immunisation Programme (UIP) of the Ministry of Health available at any government PHC, district hospital or anganwadi, and the IAP recommended schedule used by private pediatricians which adds a few additional vaccines. At birth the baby receives BCG (against tuberculosis), the first dose of oral polio vaccine (OPV), and the first dose of hepatitis B. At 6, 10 and 14 weeks the baby receives the pentavalent shot (DPT + hepatitis B + Hib), OPV, rotavirus vaccine (Rotavac in UIP), the pneumococcal vaccine PCV, and inactivated polio vaccine IPV.

At 9 months the baby receives the first MMR (or measles-rubella in UIP) along with another OPV dose and vitamin A. At 12 months the IAP schedule adds hepatitis A and a booster of certain vaccines, and at 16 to 18 months a DPT booster (often switched to DTaP if the earlier reactions were severe), an MMR booster, and another OPV booster are given. Influenza vaccine (annual from six months), typhoid (from nine months), and chickenpox (from twelve months) are part of the IAP schedule and available in private clinics for around 1000 to 2500 rupees per dose.

The UIP is free at every public health centre and is excellent and complete for the core vaccines preventing the highest-mortality diseases — WHO estimates the UIP has prevented millions of childhood deaths and is the biggest reason for the steep drop in India's infant mortality over three decades. Private clinics like Apollo, Cloudnine, Rainbow and Motherhood offer the same schedule with optional brand upgrades (Pentaxim Infanrix-Hexa Easy-Six) for around 500 to 2000 rupees consultation plus vaccine cost. Carry the printed immunisation card to every visit and ask for the next-visit date to be written on it.

Several well-meaning but harmful practices are still common in Indian families and are worth setting aside. Do not skip a scheduled dose because the baby had a mild fever or fussiness after the previous one — mild reactions are expected, are not a contraindication, and protection only comes with the full series. Do not give two doses to make up for a missed one — the schedule has specific intervals for a reason, and the pediatrician will advise on the correct catch-up plan.

Do not give aspirin to a baby for fever after a vaccine or for any reason in the first year — aspirin is associated with Reye's syndrome (a rare but serious brain and liver complication) and is replaced by safe paracetamol. Do not apply warm compress, hot oil, or any heating remedy to the injection site swelling — heat increases inflammation and worsens the swelling; if any local treatment is needed, a cool clean wet cloth gently applied for a few minutes is the right approach, and most local reactions need nothing.

Do not give antibiotics for fever after a vaccine — vaccine fever is the body's inflammatory response, not an infection; antibiotics do not help and contribute to antibiotic resistance. Do not apply turmeric ash neem paste or breast milk to the BCG site or any injection site. Do not delay vaccines because the baby has a mild cold — a low-grade illness is not a reason to postpone, and the pediatrician will check before each shot. Do not believe social media claims that natural infection is better than the vaccine — natural infections carry serious risks of complications and death.