American Health & Medical Research Association
Explainer

Babies borrow their mothers’ antibodies, and older adults’ infections may bring no fever

Immunity changes from before birth to old age, shaping who gets sick, how illness shows up and how well vaccines work. This is what each life stage faces and what helps.

By the Health Research USA editorial team

Published Last updated October 2026

Written from the sources listed below; links checked October 2026. It touches on medicines, doses or health conditions and has not yet been reviewed by a licensed clinician, so use it to prepare for a conversation with yours. How we research and review

Sources15, listed below Reading time15 minutes

Where the evidence stands

  • Strong evidence Flu and whooping cough vaccines in pregnancy protect young babiesAn infant RSV antibody shot prevents hospital staysOlder adults respond less to standard flu vaccine
  • Promising but early Drug and hormone combinations can renew the thymus
  • Not enough evidence Calorie restriction renews the thymus and prevents infections
  • Evidence says no, or unsafe A serious infection in an older adult always brings a fever

Each use is rated separately, from the scorecard in this guide. Safety cautions are spelled out in the guide itself. How we grade

On this page
  1. Newborns run on borrowed antibodies, and breast milk tops them up
  2. Pregnancy shifts a mother's defenses, and her vaccines can reach the baby
  3. RSV antibody shots hand babies ready-made protection
  4. Toddlers catch 8 to 10 colds, and each one teaches
  5. The thymus starts shrinking in childhood. T cells feel it decades later
  6. Some vaccines are built for older immune systems
  7. Infections in older adults often skip the fever
  8. Each life stage has its own risks and its own tools
  9. When to get help
  10. Questions readers ask
  11. The bottom line

Key takeaways

  • A mother’s antibodies cross the placenta mainly in the last one to two months of pregnancy, then fade from the baby’s blood over weeks to months1.
  • Most children catch about 8 to 10 colds in their first two years, and more if they’re in child care, according to the American Academy of Pediatrics10.
  • In a trial of 3,614 infants, a single RSV antibody shot cut RSV hospital stays from 2.2% of babies to 0.2% over about five months6.
  • The thymus, where T cells train, starts shrinking in early childhood, and a 2026 review says only 30% to 40% of older adults respond efficiently to flu vaccine11.
  • In older adults, infection can show up as confusion, falls or a poor appetite, and fever is missing or blunted in almost a third of cases12.

Fever is the alarm most of us expect when an infection takes hold. In older adults, that alarm often stays quiet.

Fever is absent or blunted in almost a third of older patients with an acute infection, according to a geriatrician’s 2020 commentary in Clinical Interventions in Aging12. The shorthand is “older is colder.” In older people with moderate to severe pneumonia, the average temperature over the first three days runs 0.15°C lower for every 10 years of age12.

Newborns face a different problem: they arrive with defenses they didn’t build. In the last one to two months of pregnancy, a mother’s antibodies, the proteins that tag germs, cross the placenta and guard the baby against certain diseases for its first few months1.

In between, toddlers pile up colds while they build memory, pregnancy shifts a woman’s defenses, and the thymus, where T cells train, slowly shrinks11. This guide follows those changes, and what each stage needs.

Newborns run on borrowed antibodies, and breast milk tops them up

The antibodies that cross the placenta are a type called IgG1. CDC’s Pink Book, its textbook on vaccine-preventable diseases, says they protect better against some diseases, such as measles, rubella and tetanus, than against others1.

That protection is temporary. Borrowed antibodies break down over weeks to months1. Our inference from the timing: a baby born several weeks early misses part of the main transfer window. Borrowed antibody can also blunt a live vaccine, which is why the measles shot waits until after infancy1.

Breast milk adds a second, ongoing supply. The American Academy of Pediatrics (AAP) says milk carries antibodies, and colostrum, the first milk, is especially rich in them2. Milk also carries lactoferrin and other proteins that keep inflammation in check, plus a steady stream of helpful microbes for the baby’s gut2.

The AAP links breastfeeding to fewer ear infections, bouts of vomiting and diarrhea, pneumonia, urinary tract infections and some types of meningitis, though its page gives no numbers2. For families who can’t or don’t breastfeed, our article “Breast milk and formula, without the guilt” covers healthy feeding.

Pregnancy shifts a mother’s defenses, and her vaccines can reach the baby

Pregnancy changes the immune system, heart and lungs, and CDC says those changes make severe flu more likely4. Flu in pregnancy more often leads to a hospital stay and may harm the developing baby4.

A vaccine in pregnancy uses the route nature already built. The mother makes antibodies after the shot, and they cross to the baby before birth, giving short-term protection3. Flu-vaccine antibodies can also reach a breastfeeding baby through milk, CDC says4.

The payoff shows up in babies too young for their own shots. In data CDC summarized in 2019, flu vaccination in pregnancy cut flu hospitalizations in babies under 6 months by an average of 72%5. Tdap, the whooping cough shot, lowered whooping cough in babies under 2 months by 78% and their hospital stays by 91%5.

Those are relative reductions, and CDC’s summary gave no case counts. For scale, babies under 6 months have the highest flu hospitalization risk of any children5. Mothers benefit too: a 2018 study found that vaccination cut flu hospitalizations in pregnant women by an average of 40%4.

As of October 7, 2026, CDC’s pregnancy page recommends vaccines against whooping cough, flu and RSV during pregnancy, with Tdap in every pregnancy3. Our article on immune memory explains why each pregnancy gets its own dose, and “How to read competing vaccine schedules now that the advice has split” tracks the disputes over federal advice.

Caution about anything taken in pregnancy is a protective instinct. Ask what a vaccine prevents, in numbers, and what its risks are, as you would for any medicine or herb.

RSV antibody shots hand babies ready-made protection

RSV, or respiratory syncytial virus, infects almost every child before age 27. The AAP says 2 to 3 of every 100 infected infants may need a hospital stay7. Among American children under 5, RSV causes about 80,000 hospitalizations and 300 deaths a year, according to the Pediatric Infectious Diseases Society (PIDS)9.

Families now have two options. One is Pfizer’s maternal RSV vaccine, Abrysvo, given at 32 to 36 weeks of pregnancy6. The AAP says it lowers a baby’s risk of RSV hospitalization by 57% over the first six months7. CDC and the AAP differ on the season, though: CDC’s page lists the shot from September through January, while the AAP’s runs it through March 137.

The other is a monoclonal antibody, a lab-made antibody given to the baby in one shot. Nirsevimab, from Sanofi and AstraZeneca, won FDA approval on July 17, 2023, and clesrovimab, from Merck, followed on June 9, 20258.

In the clesrovimab trial of 3,614 infants, 2.5% of babies given the antibody needed medical care for an RSV lung infection within 150 days, compared with 6.2% given a placebo, a dummy shot6. Hospital stays for RSV fell from 2.2% to 0.2%6. By our math, that’s about 4 fewer treated infections and 2 fewer hospital stays per 100 babies. Serious health problems of any cause were about equally common in both groups6.

The AAP’s parent site, updated September 2, 2026, puts the infant shots at about 80% to 90% effective against RSV hospitalization and says most babies need the maternal vaccine or the infant shot, not both7. PIDS reported that RSV hospitalizations in babies under 8 months fell 43% after the infant antibodies and the maternal vaccine came into use9.

CDC’s vaccine advisers recommended nirsevimab in August 2023 and added clesrovimab in June 2025, with no product preferred6. That 2025 vote came from the newly appointed committee whose votes a court later paused, as our vaccine schedule article explains. CDC’s pregnancy page, updated October 7, 2026, still lists an infant RSV shot as the alternative to the maternal vaccine3.

In December 2025, Reuters and NBC News reported that the FDA was launching a safety review of both antibodies, and an HHS spokesperson said the agency routinely evaluates new safety information8. PIDS said no concerning safety signal had appeared in the trials or since CDC’s 2023 recommendation9. As of October 24, 2026, we found no announced result or label change from that review.

Both antibodies are company products. CDC’s report on the clesrovimab trial doesn’t name a sponsor, but it draws in part on Merck’s own presentation of its clinical program6.

Toddlers catch 8 to 10 colds, and each one teaches

The AAP says most children have around 8 to 10 colds in their first two years, and more in child care or with school-age siblings10.

That’s what a young immune system looks like while it learns. Each new virus leaves memory behind, as our article on immune memory describes.

The thymus starts shrinking in childhood, and T cells feel it decades later

T cells coordinate the immune response and kill infected cells, and they learn their jobs in the thymus. A 2026 review in Science Advances by Jérémy Santamaria and Magali Irla says the thymus starts shrinking in the first years of life11. The process, called involution, peaks around puberty, when rising sex hormones push it along11.

As it ages, the thymus loses its orderly structure and fills with fat cells11. It turns out fewer naive T cells, fresh cells that haven’t met a germ yet. Memory T cells pile up, and the range of targets the T-cell pool can recognize narrows11.

The cost shows up when something new arrives, such as a new virus or a vaccine. The review says only 30% to 40% of older adults mount an efficient response to flu vaccine11. A related change, low-grade inflammation that rises with age, is covered in “Inflammation rises with age in modern societies, but two Indigenous groups broke the pattern.”

Can the thymus be revived? In an exploratory study of healthy men aged 51 to 65, growth hormone, metformin and DHEA together reduced thymic fat and raised naive T cells, but much of the change was gone six months after they stopped11. A two-year calorie-restriction study in middle-aged adults was also linked to thymic renewal11. The review doesn’t say whether those adults caught fewer infections11.

Some vaccines are built for older immune systems

CDC’s flu page, updated September 1, 2026, says older adults can make weaker protective responses to flu shots than healthy young people14. It lists three flu vaccines as preferentially recommended at 65 and older: a high-dose shot (Fluzone High-Dose Trivalent), a recombinant shot (Flublok Trivalent) and an adjuvanted shot (Fluad)14. An adjuvant is an added ingredient that spurs the body to respond more strongly14.

In a Hong Kong trial with CDC co-authors, adults 65 to 82 got either a standard or an improved flu vaccine by chance14. Antibodies against one flu strain rose 3.4-fold with the standard shot and 4.2- to 4.7-fold with the improved ones14. CDC cautioned that more antibody doesn’t guarantee better protection14.

Shingles grows more common with age because immunity wanes, CDC’s Pink Book says, and lifetime risk is 32% or more, about 1 in 313. The recombinant shingles vaccine, Shingrix, pairs one virus protein with an adjuvant called AS01B. In a trial of more than 30,000 people, it was 91.3% effective, in relative terms, against shingles in those 70 and older13.

Invasive pneumococcal disease, a serious bacterial infection, also climbs before 65. In 2022, it struck 13.2 of every 100,000 people aged 50 to 64 and 17.2 per 100,000 at 65 and older15. In January 2025, CDC published its advisers’ expanded pneumococcal conjugate vaccine recommendation for adults 50 and older15.

Infections in older adults often skip the fever

The 2020 commentary says that fever, cough, chest discomfort and heavy phlegm, the classic signs of a lung infection, show up in fewer than half of older adults12. Instead, illness may show up as a fall, new confusion, a poor appetite, walking less, new incontinence, trouble swallowing, or a racing heart or low blood pressure with no clear cause12. A low body temperature can signal a serious, life-threatening infection12.

The commentary also proposes a lower bar for fever. Citing in part a 2008 Infectious Diseases Society of America guideline for nursing-home residents, it would count an oral or ear temperature above 37.3°C (99.1°F, by our math) on repeat checks. A rise of more than 1.1°C (about 2°F) above the person’s usual would count too12. Treat it as one geriatrician’s proposal, not a formal standard.

That makes a baseline useful. Our suggestion: take an older relative’s temperature a few times when they’re well and write it down, so a later reading of 99.5°F has meaning.

Each life stage has its own risks and its own tools

This table is our own summary of the sources cited. Use it to frame questions for a clinician, not as a schedule.

Stage What’s changing Main risks What helps Source
Late pregnancy, for the baby Mother’s IgG crosses the placenta, mostly in the last 1 to 2 months A baby born early misses part of it (our inference) Vaccines in pregnancy add targeted antibodies 13
Pregnancy, for the mother Immune, heart and lung changes More severe flu, more hospital stays Flu vaccination; quick care for flu symptoms 4
Birth to 6 months Borrowed antibodies fade over weeks to months Highest flu hospitalization risk of any children; RSV Breast milk; maternal RSV vaccine or infant antibody shot 1257
Ages 0 to 2 Memory built one germ at a time About 8 to 10 colds; more in child care Knowing which signs need a call 10
Puberty to midlife Thymus shrinkage peaks at puberty Fewer new T cells Nothing proven yet 11
Ages 50 to 64 Shingles and pneumococcal risk rising 13.2 invasive pneumococcal cases per 100,000 in 2022 Ask about shingles and pneumococcal vaccines 1315
65 and older Weaker vaccine responses Infections without fever; 17.2 pneumococcal cases per 100,000 Improved flu vaccines; knowing your usual temperature 121415

How strong is the evidence?

Claim Evidence level Basis
Flu and whooping cough vaccines in pregnancy protect young babies Strong evidence 72% fewer infant flu hospitalizations; 78% less infant whooping cough5
An infant RSV antibody shot prevents hospital stays Strong evidence Placebo-controlled trial, 2.2% vs. 0.2%6; AAP estimate of 80% to 90%7
Older adults respond less to standard flu vaccine Strong evidence CDC and a 2026 review1114
Drug and hormone combinations can renew the thymus Promising but early One small exploratory study; gains faded within 6 months11
Calorie restriction renews the thymus and prevents infections Not enough evidence One two-year study in middle-aged adults showed thymic changes; infections not reported11
A serious infection in an older adult always brings a fever Evidence says no, or unsafe Fever is absent or blunted in almost a third of cases12

Natural health starts from respect for the body’s own defenses, and this story gives that respect plenty to work with: a mother’s antibodies shield her baby before birth and through her milk12. Companion articles weigh the evidence on nutrition and on sleep, and a later one covers exercise.

Some gaps are built into biology, such as a newborn’s untrained defenses and an older adult’s shrinking supply of fresh T cells111. Vaccines and antibody shots were designed for those gaps.

When to get help

Babies and young children

  • Call the pediatrician right away if a baby 3 months or younger has a rectal temperature of 100.4°F (38°C) or higher, even with no other signs10.
  • For babies 3 to 6 months, the AAP says you may need to call at 101°F or higher; after 6 months, at 103°F or higher10.
  • Call 911 if a child having a fever-related seizure has trouble breathing, or if the seizure doesn’t stop within 15 minutes10.
  • Call right away for pauses or trouble breathing; pale, gray or blue skin, lips or nail beds; fewer than one wet diaper every 8 hours; or much less activity or alertness7.
  • With a cold, call at the first sign of illness in a baby 3 months or younger. In older children, call for fast or hard breathing, ribs pulling in, ear pain or a daytime cough lasting more than 10 days10.

During pregnancy

  • If you’re pregnant and have flu symptoms, contact your provider right away; CDC recommends prompt antiviral treatment4.

Older adults

  • New confusion, a fall, walking less, eating less or new incontinence can signal infection, so call their clinician12.
  • One geriatrician suggests treating a temperature above 99.1°F on repeat checks, or about 2°F above their usual, as a possible fever12.
  • An unusually low temperature can signal a serious, life-threatening infection12.
  • For emergency warning signs of any kind, see “The symptoms that mean call 911 now.”

Questions readers ask

If I breastfeed, does my baby still need RSV protection?

Breast milk carries antibodies, and the AAP links breastfeeding to fewer infections2. Its RSV guidance still describes either the maternal vaccine or an infant antibody shot for most babies, usually not both7. Talk through the timing with your pediatrician or midwife.

Is an RSV antibody shot a vaccine?

Not quite. A vaccine trains the body to make its own antibodies, while the infant shot delivers ready-made ones, much like a mother’s1. CDC’s Pink Book notes that borrowed protection works right away but fades over weeks to months1.

My toddler seems sick all the time. Is something wrong?

Usually not. The AAP counts about 8 to 10 colds in the first two years as typical, with more in child care10. Call the pediatrician for the warning signs above, or if your child seems unusually sleepy or cranky10.

The bottom line

Immunity isn’t fixed. A newborn borrows its mother’s antibodies, a toddler builds memory one cold at a time, and an older adult leans on old memories as the thymus shrinks. That helps explain why infection at 85 can look like confusion or a fall instead of a fever.

Each stage has tools fitted to its gaps, from breast milk and vaccines in pregnancy to infant antibody shots and vaccines built for older bodies. Learn the warning signs for the people you care for, ask for numbers, and choose what fits your family’s health.

About this article

This article was researched using CDC’s Pink Book and other CDC pages, reports from CDC’s vaccine advisers, American Academy of Pediatrics guidance, a statement from the Pediatric Infectious Diseases Society and peer-reviewed reviews, listed below. It was reviewed in October 2026, while federal vaccine policy was still changing. It is general education, not medical, legal or financial advice.

Sources

  1. Centers for Disease Control and Prevention. “Chapter 1: Principles of Vaccination.” Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), 14th ed., page reviewed March 22, 2024. cdc.gov
  2. McCarthy C. “Breastfeeding Benefits Your Baby’s Immune System.” American Academy of Pediatrics, HealthyChildren.org, August 30, 2023. healthychildren.org
  3. Centers for Disease Control and Prevention. “Vaccine Recommendations Before, During, and After Pregnancy.” CDC, updated October 7, 2026. cdc.gov
  4. Centers for Disease Control and Prevention. “Flu & Pregnancy,” updated September 17, 2024, and “Pregnant? You Need a Flu Shot!” fact sheet, updated November 1, 2018. cdc.gov and cdc.gov
  5. Centers for Disease Control and Prevention. “Vaccinating Pregnant Women.” CDC Vital Signs, October 8, 2019. cdc.gov
  6. Advisory Committee on Immunization Practices. “Use of Clesrovimab for Prevention of Severe Respiratory Syncytial Virus-Associated Lower Respiratory Tract Infections in Infants: Recommendations of the Advisory Committee on Immunization Practices, United States, 2025.” Morbidity and Mortality Weekly Report, 2025;74(32), August 28, 2025. cdc.gov
  7. American Academy of Pediatrics, HealthyChildren.org. “RSV: When It’s More Than Just a Cold.” Updated September 2, 2026. healthychildren.org
  8. Fitch J. “Reports: FDA Launching Safety Review of Nirsevimab and Clesrovimab for RSV.” Contemporary Pediatrics, December 11, 2025. contemporarypediatrics.com
  9. Pediatric Infectious Diseases Society. “FDA Announced Review of RSV Treatments Concerns PIDS.” PIDS, December 12, 2025. pids.org
  10. American Academy of Pediatrics, HealthyChildren.org. “Children & Colds,” updated January 27, 2025, and “Fever and Your Baby,” updated September 23, 2025. healthychildren.org and healthychildren.org
  11. Santamaria JC, Irla M. “Age-Related Thymic Involution: Mechanistic Insights and Rejuvenating Approaches to Restore Immune Function.” Science Advances, 2026;12(7):eaeb2970. pmc.ncbi.nlm.nih.gov
  12. Isik AT. “Covid-19 Infection in Older Adults: A Geriatrician’s Perspective.” Clinical Interventions in Aging, 2020;15:1067–1069. dovepress.com
  13. Centers for Disease Control and Prevention. “Chapter 23: Zoster.” Pink Book, updated April 24, 2024. cdc.gov
  14. Centers for Disease Control and Prevention. “Adjuvanted Flu Vaccine,” updated September 1, 2026, and “Study Confirms That Some Flu Vaccines Cause a Stronger Immune Response Among Older Adults” (archived flu spotlight), December 12, 2019. cdc.gov and archive.cdc.gov
  15. Advisory Committee on Immunization Practices. “Expanded Recommendations for Use of Pneumococcal Conjugate Vaccines Among Adults Aged ≥50 Years.” Morbidity and Mortality Weekly Report, 2025;74(1):1–8, January 9, 2025. cdc.gov