Preventing colds and flus using science

My previous post on Vitamin C mentioned that it wasn't the best agent for preventing respiratory infections like colds, flu, and common coronaviruses (which now include COVID-19). That leads to a natural question: what is? And when you get one, what's the best way to manage it?

There is a lot of conflicting advice out there, but we're only interested in what's been definitively proven by research studies, preferably large ones, and preferably replicated reliably across multiple other trials.

So I've unleashed my auto-research engine to do another survey of scientific literature. As usual, turns out there's a huge body of research, and scanning through it was quite interesting, with some results that, in hindsight, I should have guessed were obvious, and some much less so.

So let's take a look at these studies and see what science says about dealing with respiratory illnesses.

Prevention: vaccines win, and it's not even close

For flu, the annual vaccine remains the strongest practical option, despite its unreliability. A review of 123 studies of people aged 15–64 found that seasonal flu vaccines prevented about 48% of infections in randomized trials. That success rate varied from year to year because flu strains constantly mutate and vaccine makers must guess months in advance which strains to protect against, so depending on the year, vaccines prevented as many as 55% of infections or as few as 39%. Those figures are not great, but even in the worst years vaccines still outperformed any supplement, masking, regular handwashing, or other habit.

COVID vaccines follow a similar pattern, except protection against infection fades faster than protection against serious disease. The early COVID-19 vaccines from 2022 reduced the likelihood of getting the illness by 83% in the first month after vaccination. By the fifth month, protection against infection had dropped to 22%, while protection against severe cases remained high, reducing that risk by 90%. These estimates are from the Omicron era in 2022, and they may have changed since then.

More recently, new bivalent COVID vaccines were developed, and a systematic review in 2024 showed that they reduced the likelihood of getting COVID-19 by a further 31% compared with original monovalent vaccines, with larger benefits for older adults over 50.

Unfortunately, there aren't vaccines for viruses causing common colds, like rhinoviruses or non-COVID-19 coronaviruses, so for those, we have to move on to the next tier of preventative measures.

There, handwashing seems to be the best non-pharmaceutical measure: across randomized trials involving more than 52,000 people, hand-hygiene programs reduced acute respiratory infections by about 14%. Sleeping well and managing stress also helps, though the evidence for these is less direct. In one experiment, healthy adults sleeping under seven hours were almost three times as likely to develop a cold after researchers put rhinovirus into their noses as those sleeping eight hours or more. Another similar experiment found that people in the highest stress group were more than twice as likely to develop a clinical cold as those in the lowest. Because these studies showed associations under controlled exposure, they do not necessarily guarantee that one extra hour of sleep or a meditation app will prevent your next cold, but they do suggest that managing sleep and stress can help reduce infections.

Exercise has also been tested as an intervention, but only barely. In four small trials with 281 mostly female participants, regular moderate exercise was associated with 27% fewer colds. The result is promising, but not conclusive until there are larger studies. Even so, sleep and exercise remain excellent bets for health in general; if you aren't investing in them, this is another reason to give them a try.

Supplements mostly do less than their labels imply

Vitamin D doesn't help prevent infections and is a good example of how a promising result can weaken as the evidence grows. Earlier analyses of Vitamin D trials like this one found fewer respiratory infections in those taking the supplement daily. But not long after that paper's publication, six more trials with almost 20,000 participants have been completed, and a combined 2025 meta-analysis of all 46 randomized trials up to that date concluded that there was actually no statistically reliable benefit.

Likewise, my earlier Vitamin C post showed that regularly taking vitamin C does not reduce how often the general population catches colds, although it does reduce their severity and duration when people have already been taking it daily. The exception was people under prolonged physical stress, such as marathon runners, skiers, and soldiers, for whom a daily dose of 200+ mg halved the incidence of colds. This finding applies specifically to marathon-level physical stress, so if you just go for occasional jogs or gym workouts, this finding likely doesn't apply to you.

Probiotics are showing promise, but are not yet at the stage where they can be definitively recommended. Across 42 trials in adults aged 18–65, they produced a 9% reduction in the chance of at least one respiratory infection, along with small reductions in duration and severity. But those results varied substantially, the publication bias favored positive studies, and different bacterial strains and products were mixed together. So overall, the evidence suggests that "some probiotics may do something", but does not tell you which tub of capsules to buy.

The remaining familiar supplements even are weaker. Zinc does not reliably prevent colds. Echinacea looks slightly better on paper, with a pooled 22% reduction in upper-respiratory infections, but the trials mixed different species, extracts, and doses and showed signs of publishing only positive findings; it also did not shorten infections once they began. Garlic's prevention reputation rests largely on one small, industry-linked trial that needs to be independently replicated in a larger group before making any decisions.

If you get sick, what helps you get healthier faster?

If you want to get healthier faster, and not just manage the symptoms, what you should do depends very much on the specific virus you caught, so for the best results, you'd first need to get tested.

If it turns out that you have a flu, prescription antivirals seem to be a good bet. For example, a single dose of baloxavir (a.k.a. Xofluza) taken within two days of symptom onset shortened symptoms from a median of 80 hours to 54, saving about a day of the worst symptoms. Oseltamivir (a.k.a. Tamiflu) can also help, although a meta-analysis of 15 outpatient trials found it did not clearly reduce the already-low risk of hospitalization, while increasing nausea and vomiting.

For COVID, you'd need different antivirals, though the evidence for them is a touch murkier. In one Omicron-era cohort, nirmatrelvir-ritonavir (Paxlovid) taken within five days cut hospitalization from 0.52% to 0.06%. That is an impressive relative reduction, but the absolute difference was less than half a percentage point: about 157 people had to be treated to prevent one admission. A more recent 2026 meta-analysis confirmed the antiviral combo's usefulness, especially in high-risk patients, though noted that the effect has diminished in recent years.

For a cold, I haven't seen any useful results for antivirals or any other drugs. In fact, the closest thing to a reliable treatment seems to be zinc lozenges, though the uncertainty about their effectiveness is enormous. The latest Cochrane review estimated that starting them after illness began shortened colds by about 2.4 days, but the results varied wildly across doses, salts, and formulations. Non-serious side effects such as nausea, bad taste, and mouth irritation were common. So zinc may be worth trying early if you tolerate it, but the evidence cannot identify one dependable product or dose.

Vitamin C, as mentioned before, can also reliably slightly reduce duration and severity of common colds, but only if taken daily before the cold starts. In studies where it was taken after the onset of infection, it didn't seem to make a difference.

If you just want to manage the symptoms

When the goal is just to relieve a headache or fever, ibuprofen (a.k.a. Advil or Motrin) and other non-steroidal anti-inflammatory drugs (NSAIDs) can do a great job. However, these drugs do not help you to get healthier; they only make the symptoms more bearable. They also cannot consistently help with cough and runny nose. Cold-specific evidence for acetaminophen (a.k.a. Tylenol) is surprisingly weak and inconsistent, although it remains a conventional option for pain or fever.

Combination antihistamine-decongestant-painkiller products produce small improvements in adults, but changes in individual symptoms were usually less than one point on four- or five-point scales, while dry mouth, insomnia, and dizziness became more common. That is a good argument for choosing one ingredient for the symptom bothering you instead of swallowing a three-drug bundle.

For congestion, repeated doses of older decongestants produced a small subjective improvement. Oral phenylephrine, now common in products marked “PE,” is the conspicuous exception: four trials involving 1,532 adults found it no better than placebo. The evidence is bleaker for cough. A review covering 6,496 participants found no high-quality evidence for a dependable benefit from over-the-counter suppressants, expectorants, or mucolytics. Honey modestly eased children's nighttime cough in low-quality trials, but adult evidence was thin.

Saline nasal irrigation and gargling are cheap and biologically plausible, but their most encouraging common-cold result came from an open-label pilot of only 68 adults. That pilot reported colds ending 1.9 days earlier, but duration was a secondary outcome and participants knew which treatment they received. That makes saline reasonable comfort care if you like it, not a proven two-day cure.

What doesn't work

Antibiotics are the clearest no. Most upper respiratory illnesses are caused by viruses, not bacteria, so antibiotics shouldn't even be a consideration. That has not stopped people from trying them, with unsurprising results: across 11 randomized trials, antibiotics did not improve ordinary colds or acute purulent rhinitis, and among adults with colds, adverse effects were more than twice as common. Of course, the situation changes if a cold leads to bacterial pneumonia, but for the initial cold or flu, antibiotics are a clear non-starter.

Many folk or commonly suggested remedies do not work either. Steam inhalation does not consistently improve cold symptoms. Starting vitamin C after symptoms, as mentioned earlier, does not reliably help, nor does echinacea. Elderberry failed a modern randomized test in 87 people with confirmed flu: symptoms eased after 5.3 days with elderberry and 4.9 with placebo, effectively no difference. The positive studies behind many other traditional remedies were often small, used a proprietary formulation, or failed to replicate.

So what should you actually do?

Before the season starts, get the vaccines appropriate for you, wash your hands regularly (which you should anyway), and protect your sleep. Exercise regularly because it is useful far beyond respiratory infections, and treat its cold-prevention effect as a possible bonus. If you want to try a probiotic, vitamin C, or echinacea, the likely upside is small and product-dependent; vitamin D makes more sense for correcting a deficiency than as a cold shield.

Once you are sick, you can try to get tested to see whether it might be flu or COVID, in which case antivirals may help. For an ordinary cold, you can try treating the symptom that is actually bothering you: an NSAID for aches, a real decongestant for congestion, perhaps zinc lozenges if you accept the uncertain formulation and side effects. Skip oral phenylephrine, antibiotics, steam, Vitamin C, and expensive immune-boosting cocktails.