Immunity: what research links to it, and what is marketing

"Boost your immunity" is a phrase with an assumption built in: that there is a dial called immunity, and that you can turn it up.

The research does not look like that. Studies pick one measurable outcome and ask what goes with it: whether people caught a cold after a controlled exposure, how many days the cold lasted, how many antibodies appeared after a vaccine. These are different questions with different answers, and a factor that moves one of them often says nothing about the others. This is informational material, not medical advice. Questions about supplements, medicines or vaccines belong with a doctor or pharmacist, and so do frequent or severe infections.

Below: what has some evidence of association, what the two best-tested supplements showed, and what the whole pile does and does not add up to.

Why "immunity" is not one thing

The immune system is a set of cooperating parts: barriers such as the lining of the nose, fast innate responses, slower learned memory, and the signalling that ties them together. A single word for all of that is convenient for packaging. It is a poor description of what a study can see.

So the useful question is always narrower. Which outcome was measured, in whom, and was the factor assigned by the researchers or only observed? That last part decides how much a result can carry, and it is why each finding below is labelled with its type.

Sleep and stress: two links already covered

Two of the better-known associations have their own articles, so only the shape of them here.

On sleep: in a virus-challenge setting, people who had slept less beforehand caught colds more often. Separately, shorter measured sleep went with a lower antibody response to a hepatitis B vaccine in 125 adults aged 40 to 60 (Prather et al., 2012). That one is observational, and the details and limits are in sleep and catching a cold.

On stress: in 394 volunteers who were given respiratory viruses, rates of colds rose with psychological stress, from about 27% to about 47% across stress levels (Cohen et al., 1991). The stress was measured by questionnaire, not changed by the researchers. Fatigue and getting sick goes through what that study can and cannot say about a tired week.

Social ties: the newer part

This one is less familiar. In the 1997 study, 276 healthy volunteers aged 18 to 55 first reported which of 12 types of social ties they had: spouse, parent, friend, workmate, member of a social group and so on. Then they were given nasal drops containing one of two rhinoviruses and stayed in quarantine. A cold counted only if the illness came with a verified infection (Cohen et al., 1997).

People with more types of ties were less susceptible to colds. They also produced less mucus, cleared their nasal passages more effectively and shed less virus. The effect grew step by step with the diversity of the network, and the adjusted relative risk was 4.2 when the fewest types of ties (1 to 3) were compared with the most (6 or more). The analysis accounted for antibodies at the start, virus type, age, sex, season, body mass index, education and race.

Note what was counted: types of ties, not the number of friends. Someone with a spouse, a colleague and a sports club scored higher than someone with twenty contacts of a single kind.

The design is half experimental and half not. The virus was given to everyone, which is the strong part. The ties were only reported, never assigned, so nobody knows whether widening a person's circle would change anything. The authors also found that smoking, poor sleep quality, alcohol abstinence, low vitamin C intake, higher catecholamines and introversion were each linked with more colds, but they explained the link with social ties only in part. It is an association with a plausible biology around it, and no recipe.

Age: the one factor you cannot adjust

Aging brings a remodeling of the immune system, which over time leads to a decline in its efficacy: more vulnerability to infectious disease and weaker responses to vaccination. The hallmarks the review describes are fewer naïve cells in the blood, relatively more memory cells, and chronic low-grade "inflamm-aging" (Aiello et al., 2019).

That is a review's description of a process, so it is background, not a measurement of your own. It is also a reason that a result found in 25-year-old volunteers need not carry over to a 70-year-old.

Vitamin C: a large review with a split answer

The Cochrane review of vitamin C pooled placebo-controlled randomized trials, using at least 0.2 g a day (Hemilä & Chalker, 2013). Its results depend on who was studied.

• In general community trials of regular supplementation (10,708 participants), the pooled risk ratio for catching a cold was 0.97 (95% CI 0.94 to 1.00). That is essentially no reduction.
• In five trials with 598 marathon runners, skiers and soldiers on subarctic exercises, the risk ratio was 0.48 (95% CI 0.35 to 0.64).
• Regular supplementation shortened colds by 8% in adults (3% to 12%) and by 14% in children (7% to 21%).
• Taking vitamin C only after symptoms started (therapeutic trials, 3,249 episodes) showed no consistent effect on duration or severity.

The authors concluded that routine vitamin C supplementation is not justified for the general population, and that it may be useful for people exposed to brief periods of severe physical exercise. That is the review's conclusion, not a recommendation. Whether any of it applies to you is a question for your doctor or pharmacist.

Echinacea: products that differ too much to pool cleanly

A second Cochrane review covered 24 double-blind trials with 4,631 participants. The products varied widely in species, plant parts and extraction, so "echinacea" in one trial is not the same preparation as in another. None of the 12 prevention comparisons found a statistically significant difference, though a post hoc pooling suggested a 10% to 20% relative risk reduction. Of seven treatment trials reporting how long a cold lasted, only one showed a significant effect (Karsch-Völk et al., 2014).

The authors wrote that these products "have not here been shown to provide benefits for treating colds". A weak benefit from some products remained possible, with effects of questionable clinical relevance. A post hoc result is a hint to test in the next trial, not a finding.

Other supplements are outside this article.

What none of this adds up to

Put the pieces side by side and the picture is uneven. Sleep, stress and social ties are associations: the virus was given by researchers in some of these studies, but sleep, stress and ties were observed, not assigned. Age is a background condition. Vitamin C has a randomized answer, and it is mostly "no reduction" for the general population with an exception in a narrow group. Echinacea has a randomized answer that is mostly "not shown".

Three points follow.

1. Nothing here is a lever. Each result links a factor with an outcome in a particular group. None says that changing the factor changes your next cold.
2. Observational and randomized evidence answer different questions. People who sleep well differ from people who sleep badly in ways nobody measured. Randomization is the tool that handles that, and it exists for the supplements but mostly not for the lifestyle factors.
3. The outcomes differ. Catching a cold, a cold's length and an antibody count are separate measurements, and none of them is "immunity".

No study here gives you an immunity score, and the app does not either. MeteoHealth reads sleep, HRV and resting heart rate from Apple Health and logs local outdoor weather next to your symptom entries, with nine fixed symptoms (headache, fatigue, drowsiness, dizziness, anxiety, irritability, nausea, joint pain and insomnia). It looks for correlations in your own data with Pearson, ANOVA and false-discovery-rate correction. It does not detect, diagnose or predict infections, and it has no cough, sore throat or fever entries. What you can watch is your own sleep and fatigue trend. For the season itself, see why we get sick in winter, and for a common cold-weather belief, does getting cold give you a cold.