Immunity and cold hardening: what works, according to research
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In short
You can’t “boost immunity” with a single pill. According to research, five things work: a flu shot, at least 7 hours of sleep, regular cardio, hand washing, and vitamin D if you are low on it. Cold exposure, gargling, probiotics and sauna give a small or uncertain effect. Garlic, ginger, vitamin C “for everyone” and “immunomodulators” do almost nothing for prevention.
How to read the levels of evidence
Each item below has one of four levels:
- Strong evidence — a systematic review or meta-analysis of many RCTs; the effect is consistent.
- Moderate — several RCTs or one large one; there is an effect, with caveats.
- Weak — one small RCT, observational data, or a borderline effect.
- Doesn’t work — tested, no effect.
An RCT is a randomised controlled trial: people are split into groups at random, so the difference between the groups can be put down to the intervention itself rather than to the participants’ lifestyle.
What works
| What | Effect in numbers | Evidence | Source |
|---|---|---|---|
| Flu shot once a year, September–November | In healthy adults, 2.3% got flu without the vaccine and 0.9% with it: about 71 people need to be vaccinated to prevent one case of flu | Strong | Cochrane, Demicheli 2018 |
| At least 7 hours of sleep | People who slept less than 6 hours got sick 4.2 times more often after cold-virus nose drops than those who slept more than 7 | Moderate | Prather 2015, Sleep (164 people) |
| Cardio on 5 or more days a week: walking, running, swimming | 43% fewer days with respiratory infections and 32–41% milder symptoms than in people active on 1 day a week or less | Moderate | Nieman 2011, BJSM (1,002 people, observational) |
| Hand washing | About 14% fewer acute respiratory infections | Strong | Cochrane, Jefferson 2023 |
| Vitamin D every day | 12% fewer respiratory infections overall; 70% fewer in people with a deficiency (below 25 nmol/L). Only daily or weekly doses work | Strong | Martineau 2017, BMJ (25 RCTs, ~11,000 people) |
| Vitamin D by injection or 100,000 IU once a month | Doesn’t protect against respiratory infections: in the ViDA trial (5,110 people, ~3 years) monthly doses made no difference compared with placebo. A sharp peak switches on enzymes that break down the vitamin’s active form | Doesn’t work | Camargo 2020 (ViDA); Martineau 2017 |
| Zinc within the first 24 hours of symptoms — treatment, not prevention | Zinc acetate lozenges, 75 mg a day or more, shorten a cold by about a third | Moderate | Hemilä, meta-analyses |
What helps a little, or isn’t certain
| What | Effect | Evidence | Source |
|---|---|---|---|
| Cold shower for 30–90 seconds at the end of a warm one, 30 days | 29% less sick leave, but no fewer days of illness: people are ill just as long, they only cope better | Moderate | Buijze 2016, PLOS One (3,018 people) |
| Gargling with water 3 times a day | 36% fewer respiratory infections over 60 days; the effect is borderline significant, with no placebo | Weak | Satomura 2005 (387 people) |
| Probiotics | About a quarter fewer people with at least one respiratory infection; low certainty | Weak | Cochrane 2022 |
| Regular sauna | Fewer colds in a small RCT; in a Finnish cohort, 4–7 sessions a week went with fewer cases of pneumonia | Weak | Ernst 1990; Kunutsor 2017 |
| Air humidity of 40–60% | At 43% humidity or higher, airborne flu virus quickly loses its infectivity; at 23% or lower it keeps it | Weak | Noti 2013 (laboratory) |
| Less chronic stress | The risk of a cold rises with the level of stress | Moderate | Cohen 1991, NEJM |
| Garlic every day | One RCT: fewer colds (24 vs 65), but the same number of days ill | Weak | Cochrane 2014 |
What doesn’t work for prevention
| What | Why | Evidence |
|---|---|---|
| Vitamin C “for everyone” | Doesn’t reduce the number of colds in ordinary people, only shortens them by 8%. The exception is heavy physical strain (marathon runners, soldiers in the cold): 50% fewer colds there | Doesn’t work (strong evidence for athletes) |
| Ginger, lemon, honey “for immunity” | No RCTs on prevention. Hot drinks ease symptoms and honey eases a cough, but they don’t protect against illness | Doesn’t work |
| “Immunomodulators” (Arbidol, Ergoferon and similar) | No good-quality RCTs confirm that they prevent illness | Doesn’t work |
| Antibiotics for colds and flu | Colds and flu are caused by viruses, and antibiotics don’t act on viruses | Doesn’t work |
| Forcing yourself through ice-water dousing | No better evidence than for a mild cold shower, and there is a risk of hypothermia | Doesn’t work |
What this looks like in practice
All year round. Sleep 7–8 hours and go to bed at the same time. At least 150 minutes of cardio a week: 30 minutes on at least 5 days — walking, running, swimming. Vitamin D every day, with a meal that contains some fat. After being outside, wash your hands with soap for 20 seconds and gargle with water. Ablution (wudu) five times a day is a good base, but soap after being outside is still needed. A cold shower for 30–60 seconds at the end of a warm one — to feel awake and to get through illness more easily, not as protection. Don’t smoke, and don’t sit next to people who smoke.
Before the season, in September–October. Get a flu shot before November: protection takes about two weeks to build up. Have your vitamin D tested (25(OH)D): if it is below 50 nmol/L (20 ng/mL), a doctor chooses the dose; the usual prevention guideline for adults is 1,000–2,000 IU a day, with a repeat test after 2–3 months. Keep zinc acetate lozenges at home, so you can start on the very first day of symptoms. Check the humidifier and the hygrometer.
During the season, November to March. Bedroom humidity at 40–60%, and airing the room 2–3 times a day. Fewer crowds at the peak of an epidemic, especially with small children. On the first day of symptoms: zinc, rest, plenty of warm drinks — and don’t go to work or the mosque while ill.
Which vitamin D to choose
- D3, every day or once a week. Not one-off “loading” doses and not monthly injections: they don’t protect (see above).
- A medicine, not a dietary supplement. The dose in a registered medicine is checked as strictly as in tablets; in supplements the actual dose may differ from the label.
- Water-based drops are easy to dose and have no gelatine shell, whose source (pork or beef) is often not stated for capsules.
- No vitamin A in it: it builds up with long-term use and isn’t needed for prevention.
- Check the ingredients on the leaflet: water-based D3 solutions usually contain the preservative benzyl alcohol — this is not ethanol.
When to see a doctor right away
- A temperature of 38.5 °C or higher for more than 3 days, or a second “wave” of fever.
- Shortness of breath, chest pain, confusion, bluish lips.
- In a child: lethargy, refusing to drink, rarely passing urine.
- For flu, antivirals (oseltamivir) are prescribed by a doctor, preferably within the first 48 hours.
Sources
- Demicheli V. et al. Vaccines for preventing influenza in healthy adults. Cochrane, 2018 — https://pubmed.ncbi.nlm.nih.gov/29388196
- Prather A. et al. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep, 2015
- Nieman D. et al. Upper respiratory tract infection is reduced in physically fit and active adults. BJSM, 2011 — https://bjsm.bmj.com/content/45/12/987
- Jefferson T. et al. Physical interventions to interrupt or reduce the spread of respiratory viruses. Cochrane, 2023
- Martineau A. et al. Vitamin D supplementation to prevent acute respiratory tract infections. BMJ, 2017 — https://www.bmj.com/content/356/bmj.i6583
- Camargo C. et al. Effect of monthly high-dose vitamin D supplementation on acute respiratory infections in older adults (ViDA). Clin Infect Dis, 2020
- Hemilä H. Zinc lozenges and the common cold: a meta-analysis. JRSM Open, 2017
- Buijze G. et al. The Effect of Cold Showering on Health and Work. PLOS One, 2016
- Satomura K. et al. Prevention of upper respiratory tract infections by gargling. Am J Prev Med, 2005
- Zhao Y. et al. Probiotics for preventing acute upper respiratory tract infections. Cochrane, 2022 — https://www.cochrane.org/CD006895/probiotics-preventing-acute-upper-respiratory-tract-infections
- Noti J. et al. High humidity leads to loss of infectious influenza virus. PLOS One, 2013
- Cohen S. et al. Psychological stress and susceptibility to the common cold. NEJM, 1991
- Hemilä H., Chalker E. Vitamin C for preventing and treating the common cold. Cochrane, 2013
- Lissiman E. et al. Garlic for the common cold. Cochrane, 2014 — https://www.cochrane.org/CD006206/ACUTE_garlic-common-cold
- Sanu A., Eccles R. The effects of a hot drink on nasal airflow and symptoms of common cold and flu. Rhinology, 2008
Not medical advice
This is a review of research, not a prescription. Vitamin D doses and any medicines — only with a doctor, especially for children and people with chronic illnesses.