The colder weather brings Hong Kong's peak flu season, with viruses spreading like wildfire through schools, offices and public spaces. At low temperatures, the flu virus' outer covering hardens to a rubbery gel that can protect it as it passes from person to person, according to a 2008 study by the US National Institutes of Health. When it's warmer, the gel melts, and isn't tough enough to shield the virus against the elements. The Hong Kong government has made a push to improve immunity this flu season, with people most susceptible - children, the elderly, medical officers and those with chronic illness - encouraged to get free or subsidised injections. There are many alternative remedies that claim to prevent and treat flu. We look at whether they really work. Echinacea Echinacea is a collection of plant species used for treating infections, inflammation and other ailments. Clinical trials on its efficacy have produced conflicting results. A recent study by the Cardiff University Common Cold Centre - reportedly the largest ever, involving 750 people - showed that taking three daily doses of Echinaforce, a common form of the herb extract, for four months reduced the number of colds. The duration was also reduced by an average of 26 per cent. The study was partially funded by A. Vogel, makers of Echinaforce, and published in the journal Evidence-Based Complementary and Alternative Medicine . In another study, published in 2010 in the Annals of Internal Medicine , University of Wisconsin-Madison researchers found that Echinacea had little impact on the common cold. The trial, involving more than 700 people aged 12 to 80, showed that patients who got Echinacea saw the duration of their cold reduced by seven to 10 hours. "But this did not have a large impact on the course of the cold, compared to blinded placebo or to no pills," says lead researcher Bruce Barrett. He adds there were no side effects, so there is no reason sufferers should stop using it. Echinacea is thought to work by stimulating the immune system. Elderberries Elderberry, or elder, has been used historically as a topical treatment for wounds, and taken orally for cold and flu. According to the University of Maryland Medical Centre, chemicals in elderflowers and berries help reduce swelling in mucous membranes and help relieve nasal congestion. A study review of the effect and efficacy of elderberry fruit ( Sambuci fructus ) was published in Phytotherapy Research in 2009. A number of studies - several in vitro, one on chimpanzees and another two in humans - suggested the elderberry extract Sambucol may be useful against viral influenza. However, the researchers add that the "effects of elderberry fruit preparations from studies should be backed by more rigorous studies before they are recommended." Vitamin C Affordable and found naturally, it is thought to help immune defence due to its antioxidant properties. But since the discovery of vitamin C in the 1930s, its efficacy in treating lung infections and colds has been questioned. According to a review of 30 studies by the Australian National University and the University of Helsinki, unless you run marathons, you won't get much protection from colds by taking a daily dose. The studies were conducted over several decades and included more than 11,000 people who took daily doses of at least 200mg. The review also showed that vitamin C doesn't reduce a cold's length or severity. However, the scientists found that people exposed to periods of high stress - marathon runners, skiers and soldiers on sub-arctic exercises - were 50 per cent less likely to catch a cold if they took a daily dose of vitamin C. Vitamin C is not considered toxic, but high doses can cause stomach upset. Individuals prone to kidney stones should be cautious about taking high levels over long periods. Zinc According to a review of 15 clinical trials published in The Cochrane Library in February 2011, zinc supplements reduce the severity and duration of illness caused by the common cold. It was found that zinc syrup, lozenges or tablets taken within a day of the onset of cold symptoms reduce the severity and length of illness. At seven days, more of the patients who took zinc had cleared their symptoms compared to those who took placebos. Children who took zinc syrup or lozenges for five months or longer caught fewer colds and took less time off school. However, lead researcher Meenu Singh notes that it is difficult to make a general recommendation regarding zinc intake, as little is known about the optimum dose, formulation or length of treatment. Zinc is said to work by coating the virus and stopping it from entering the body through the nasal passages, and from replicating.