流感在冬天傳播的真實原因
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2015年 10月 27日
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每當夏去秋來，落葉飄零，它都會準時出現。隨著溫度降低，陽光漸弱，鼻涕也會隨之而來。如果病情不重，那就隻是一場感冒，會讓我們的喉嚨感覺有如刀刮過一樣；如果你不幸病情較重，整個身體都會發高燒，四肢也會感覺疼痛，這種情況可能持續一周甚至更長時間。這說明我們得了流感。
流感季節總是很有規律，而且總會影響很多人。但即便如此，科學家對於寒冷的天氣為什麽有助於流感病菌的傳播卻知之甚少，這著實令人意外。
然而，過去5年間，他們終於找到了答案，這或許有助於阻止流感的大範圍爆發——這一切的核心都源自你打出的噴嚏在空氣中的傳播方式。
人類的確應當盡早加深對流感的認識；每個流感季節，全世界都有多達500萬人患病，其中約25萬人會因此喪命。流感的破壞力之所以如此巨大，部分原因在於流感病毒的變異速度極快，往往令我們的身體對下一季的菌株措不及防。
“我們體內的抗體已經無法識別這些病毒——所以我們失去了免疫力。”布裏斯托大學的簡·梅茨(Jane Metz)說。這也加大了開發疫苗的難度，雖然完全可以為每一種菌株都設計一種新的疫苗，但政府往往無法說服足夠的人注射疫苗。
倘若能夠更好地了解流感為什麽在冬季流行，到了夏季卻銷聲匿跡，醫生便可找到簡單的方法來抑製它的傳播。之前的理論都集中於我們自身的行為。我們冬天會在室內停留更長時間，因而會與可能攜帶病菌的人更近距離的接觸。
到了冬天，我們也更有可能乘坐公共交通工具，例如——當緊靠著唾沫飛濺的路人時，當車窗上的霧氣混入了他們的咳嗽或噴嚏時，我們便很容易置身於流感傳播的引爆點之中。
還有一個流行的觀點則把分析重點集中於我們的生理機能：寒冷的天氣會導致你的身體喪失對傳染病的抵抗力。由於冬天日照時間縮短，我們體內的維生素D含量可能降低。由於維生素D有助於加強我們的免疫係統，因此這種營養物質的缺乏導致我們更容易患傳染病。
另外，在冷空氣中呼吸時，鼻腔內的血管也有可能收縮，避免損失熱量。這有可能阻礙白細胞(對抗病菌的鬥士)到達鼻粘膜，使之無法殺死我們吸入的病毒，讓病菌得以在不知不覺間繞過人體的防禦係統。(或許正因如此，所以當我們濕著頭發出門時很容易感冒。)
雖然這些因素都在流感的傳播中扮演了一定的角色，但分析表明，這些理論依然無法完全解釋每年都會出現的流感季節。事實上，真正的答案或許隱藏在我們呼吸的隱形空氣中。由於熱力學定律，冷空氣攜帶的水蒸汽將會減少，並在到達“露點”後變成雨水降落下來。
所以，盡管到了冬天，外麵的天氣看起來更潮濕了，但由於空氣濕度降低了，所以空氣本身反而更加幹燥。過去幾年的一係列研究表明，這些幹燥的環境似乎為流感病毒的生長和傳播創造了絕佳的環境。
例如，科學家已經通過實驗研究了流感在豚鼠之間的傳播途徑。在潮濕的空氣中，流感病毒難以積聚勢頭，而在幹燥的空氣中卻會像野火般迅速蔓延。
將最近30年的氣候記錄與健康檔案進行對比後，哥倫比亞大學的傑弗裏·沙曼(Jeffrey Shaman)和他的同事發現，流感爆發幾乎總是伴隨著空氣濕度的下降。
事實上，這兩個圖形的重疊度極高，“幾乎可以用一個圖形完全覆蓋另一個。”梅茨最近與亞當·費恩(Adam Finn)共同為《傳染病期刊》(Journal of Infection)評估了所有的證據。這一發現已經得到了反複的驗證，包括2009年爆發的豬流感。
這似乎違反直覺——我們通常認為，潮濕的天氣容易使人生病，而不是為我們提供保護。
要理解背後的原因，就需要了解咳嗽和打噴嚏的動力學特性。當我們因為得了感冒而咳嗽或打噴嚏時，會從鼻子和嘴巴噴出許多微粒。
在潮濕的空氣中，這些微粒可能相對較大，很容易落到地麵上。但在幹燥的空氣中，這些微粒會分解成更小的微粒——最終小得足以在空中漂浮好幾個小時，甚至好幾天。(這有點像把噴頭調到最細的那一檔上產生的噴霧。)
正因如此，到了冬天，我們吸入的空氣中就會混入最近進入房間的所有人通過呼吸道排出的死細胞、粘液和病毒。
另外，空氣中的水蒸氣本身似乎也會對病毒構成威脅。通過改變一團粘液的酸度或鹽度，濕潤的空氣或許就可以瓦解病毒的表麵，使之失去攻擊人體細胞的常用武器。相比而言，病毒能在幹燥的空氣中漂浮好幾個小時，直至被我們吸入呼吸道，並在我們喉嚨的細胞裏安家。
但也有一些例外情況。雖然飛機上的空氣通常都很幹燥，但似乎並不會增加患流感的風險——或許是因為空調本身可以過濾病菌，使之無法流通。
而且，盡管幹燥的空氣似乎能在歐洲和北美的溫帶地區促進流感的傳播，但在更靠近赤道的地方卻產生了一些矛盾的結果，表明病菌在那些地方的活動方式可能略有不同。
有一種觀點認為，在特別溫暖濕潤的赤道氣候中，病毒最終會貼附在室內的更多表麵上。所以，盡管流感病毒不能在空氣中輕易生存，但它卻能存活於你所觸及的任何一個表麵，使之更有可能通過手進入我們的口腔。
但至少在北半球，可以根據這些發現設計一種簡單的方式，在病菌仍然漂浮在空氣中的時候將其消滅。當泰勒·科普(Tyler Koep)就職於明尼蘇達州羅切斯特市的梅約診所時，他曾經估計，隻要在學校裏使用空氣加濕器一個小時，就能殺死30%漂浮在空氣中的流感病毒。
還可以采取其他一些類似的措施，例如在醫院大廳或公共交通工具等流感傳播的熱點地區潑灑冷水。“這可以抑製因為病毒的變異而每隔幾年爆發一次的嚴重流感疫情。”他說，“疫情會導致企業停工、學校停課、健康受損，潛在影響十分巨大。”
沙瑪目前正在展開進一步的嚐試，但他認為，應該采取更加微妙的平衡措施。“盡管提高濕度可以降低流感病毒的存活率，但其他的病原體卻會因為濕度的升高而更加活躍。”他說，“所以在加濕過程中必須保持謹慎——這並不是有利無害的。”
科學家們都在強調，疫苗和良好的個人衛生仍是保護自己的最好方式；使用水蒸氣殺死病菌隻是額外開辟的一條戰線。但當你要對付像流感病毒那樣善變而無處不在的敵人時，就需要動用所有可以使用的武器。
請訪問 BBC Future 閱讀 英文原文。
（責編：友義 )
吃與活注： 英文原文有原始研究的鏈接。
It begins as surely as the leaves dropping off the trees. As the mercury drops and the sunlight fades, the sniffles set in. At best, it’s just a cold that leaves us with the strange feeling that we’ve swallowed a cheese grater; if we’re unlucky, our body is wracked with a high fever and aching limbs for up a week or longer. We have flu.
The flu season arrives so predictably, and affects so many of us, that it’s hard to believe that scientists have had very little idea why cold weather helps germs to spread. Over the last five years, however, they have finally come up with an answer that might just offer a way to stem the tide of infection – and it revolves around a rather grim fact about the ways that your sneezes linger in the air.
The fact that it is simply colder in winter can’t explain the yearly flu season
A new understanding of influenza couldn’t come quickly enough; worldwide, up to five million people catch the illness each flu season, and around a quarter of a million die from it. Part of its potency comes from the fact that the virus changes so quickly that the body is rarely prepared for the next season’s strain. “The antibodies we’ve built up no longer recognise the virus – so we lose our immunity,” says Jane Metz at the University of Bristol. It also makes it harder to develop effective vaccines, and although you can engineer a new jab for each strain, governments often fail to persuade enough people to take it up.
The hope is that by understanding better why flu spreads in winter, but naturally fades in summer, doctors could find simple measures to stop its spread. Previous theories had centred on our behaviour. We spend more time indoors in the winter, meaning that we’re in closer contact with other people who may be carrying germs. We’re more likely to take public transport, for instance – and as we’re pressed against spluttering commuters, misting up the windows with their coughs and sneezes, it’s easy to see how this could send us over a tipping point that allows flu to spread through a population.
Without much sunlight, we may run low on Vitamin D, weakening the immune system
Another popular idea concerned our physiology: the cold weather wears down your body’s defences against infection. In the short days of winter, without much sunlight, we may run low on Vitamin D, which helps power the body’s immune system, making us more vulnerable to infection. What’s more, when we breathe in cold air, the blood vessels in our nose may constrict to stop us losing heat. This may prevent white blood cells (the warriors that fight germs) from reaching our mucus membranes and killing any viruses that we inhale, allowing them to slip past our defences unnoticed. (It could be for this reason that we tend to catch a cold if we go outside with wet hair.)
While such factors will both play some role in transmission, analyses suggested that they couldn’t completely explain the yearly emergence of flu season. Instead, the answer may have been lying invisible in the air that we breathe. Thanks to the laws of thermodynamics, cold air can carry less water vapour before it reaches the “dew point” and falls as rain. So while the weather outside may seem wetter, the air itself is drier as it loses the moisture. And a steady stream of research over the past few years has shown that these dry conditions seem to offer the perfect environment for the flu virus to flourish.
Lab experiments, for instance, have looked at the way flu spreads among groups of guinea pigs. In moister air, the epidemic struggles to build momentum, whereas in drier conditions it spreads like wildfire. And comparing 30 years’ worth of climate records with health records, Jeffrey Shaman at Columbia University and colleagues found that flu epidemics almost always followed a drop in air humidity. In fact, the overlap of the graphs was so close, “you could pretty much put one on top of each other,” says Metz, who together with Adam Finn, recently reviewed all the evidence for the Journal of Infection. The finding has now been replicated many times including analyses of the 2009 Swine flu pandemic.
In winter, you are breathing a cocktail of dead cells, mucus and viruses from everyone who has visited the room recently
Should I wear a facemask?
What studies say
Anytime you walk into a public place, you are breathing in a fine mist of other people’s coughs and sneezes – which can hang around in the air for days. Face masks are a common precaution to stop you breathing in the germs – but do they work?
To find out, one Australian study targeted the families of people turning up at hospital with influenza. Relatives who wore surgical masks were 80% less likely to become infected themselves.Although later papers have mostly confirmed the results, it seems that it is only effective alongside hand-washing and generally good hygiene. Otherwise, it’s a little like locking all your windows while leaving the front door wide open – you are missing the most obvious line of defence.
That’s counter-intuitive – we normally think that the damp makes us ill, rather than protects us from disease. But to understand why, you need to grasp the peculiar dynamics of our coughs and sneezes. Any time we splutter with a cold, we expel a mist of particles from our nose and mouths. In moist air, these particles may remain relatively large, and drop to the floor. But in dry air, they break up into smaller pieces – eventually becoming so small that they can stay aloft for hours or days. (It’s a bit like the mist you get when you turn a hose pipe to its finest spray.) The result is that in winter, you are breathing a cocktail of dead cells, mucus and viruses from anyone and everyone who has visited the room recently.
What’s more, water vapour in the air seems to be toxic to the virus itself. Perhaps by changing the acidity or salt concentration in the packet of mucus, moist air may deform the virus’s surface, meaning that it loses the weaponry that normally allows us to attack our cells. In contrast, viruses in drier air can float around and stay active for hours – until it is inhaled or ingested, and can lodge in the cells in your throat.
There are some exceptions to the general rule. Although the air on aeroplanes is generally dry, it does not seem to increase the risk of catching influenza – perhaps because the air conditioning itself filters out any germs before they have a chance to circulate. And although the dry air seems to fuel the spread of flu in the temperate regions of Europe and North America, some contradictory results suggest the germs may act somewhat differently in more tropical areas.
In particularly warm and wet conditions, the virus may end up sticking to more surfaces within a room
One explanation is that in particularly warm and wet conditions of a tropical climate, the virus may end up sticking to more surfaces within a room. So although it can’t survive in the air so well, the flu virus could instead be thriving on everything that you touch, making it more likely to pass from hand to mouth.
But in the northern hemisphere at least, these findings could offer a simple way to kill the germs while they are still hanging in the air. Tyler Koep, then at the Mayo Clinic in Rochester, Minnesota, has estimated that simply running an air humidifier in a school for one hour could kill around 30% of the viruses flying around the air. Similar measures could (almost literally) pour cold water on other disease hotspots – such as hospital waiting rooms or public transport. “It would be a way of curbing the large outbreaks that occur every few years as the flu virus changes,” he says. “The potential impact in the cost of work days missed, schools days missed, and healthcare, would be substantial.”
Shaman is now working on further trials, though he thinks that it will involve a tricky balancing act. “Though higher humidity is associated with lower survival rates for influenza, there are other pathogens, such as pathogenic mould, that thrive at higher humidity,” he says. “So care must be taken with humidification – it's not solely beneficial.”
The scientists are keen to emphasise that measures like vaccines and good personal hygiene are still the best ways to protect yourself; using water vapour to kill the germs would just offer an additional line of attack. But when you are dealing with an enemy as mercurial and pervasive as the flu virus, you need to use every possible weapon in your arsenal.
