Italian COVID-19 patients are isolated in a makeshift hospital to prevent them causing outbreaks in ordinary hospitals.
This page aims to summarise our understanding of the current science on key questions about COVID-19 (as of 3 April, 2020), as best we can given the state of the evidence and the fast moving situation. We provide more explanation as well as sources in the footnotes.
The most common reported symptoms are cough (appearing in about 80% of confirmed cases – meaning those who have been tested and found to be infected with the virus) and fever (80%-90%).1 Many also experience shortness of breath, usually later in the disease progression. Diarrhea and other GI symptoms have also been seen in some patients.2 Nasal congestion and runny nose seem uncommon (<5%).3 Anecdotally, loss of the sense of taste or smell have also been reported.4
Once someone is infected, it seems to typically take ~7 days for symptoms to develop. One study with a large sample size found that for 11.5% of confirmed cases it took more than 14 days. 5
According to initial data from China, around 81% of confirmed cases are ‘mild’ (though can still involve pneumonia), 14% are severe (requiring hospitalisation), and 5% are critical.6 A large proportion of people infected with the virus have mild symptoms, and around 20% may have no symptoms,7 though there is not reliable data here.
Most current estimates of the fraction of infected people (rather than people with confirmed cases) who die from the disease (the ‘IFR’) seem to be between 0.1% and 2%.8 The fraction will vary based on whether healthcare capacity is overwhelmed, as well as the age and health of the population. One reason estimates are uncertain is that we don’t know how many mild and symptom-free cases there are, which usually go unconfirmed; if there are more, the IFR is lower.
Risks of long-term health effects are unknown because not enough time has passed, though there are concerns that some portion of COVID-19 patients (perhaps those with more severe cases) might face ongoing issues like reduced lung capacity.9
Who is most at risk?
Children can catch and spread the virus, though they may be less likely to, and they rarely have serious symptoms.10
Men seem to be about 50% more likely to die of the disease than women on average.11
Age is a large risk factor for mortality, as suggested in the set of estimates in the table below, used by researchers at Imperial College London. The estimates are for infections (rather than for confirmed cases) and based on data from the early phase of the disease in China:
How COVID-19 spreads is not fully understood, but most experts seem to think droplets from coughs and sneezes are most important. ‘Aerosols’ — smaller droplets that can be expelled even while talking and hang in the air longer — may also be significant.12
This means the main ways you can become infected are by being close to someone talking, coughing, or sneezing, and by touching an infected surface and then touching your face. Sources commonly recommend staying six feet away from others, but this just is a best guess for how far viral particles can travel through the air, and further is of course safer. On plastic and stainless steel, virus particles can remain viable for three days or more; they remain viable for less time on copper and cardboard.13
It’s thought that people can be infectious perhaps one or two days before they develop symptoms, though they’re less infectious than when they have symptoms.14 Totally asymptomatic people (including children) may also be infectious. It’s currently debated how much transmission is driven by asymptomatic and very mild cases.15
It’s unknown how long people stay infectious after they develop symptoms. The CDC recommends home isolation for seven days after symptoms start and three days after they resolve. By contrast, the WHO recommends isolation for 14 days after symptoms resolve.16
The number of infections is higher than the number of confirmed cases everywhere. By how much depends on how extensive testing is, which varies extremely widely – it seems plausible some countries could be capturing up to half of infections while others are capturing as little as 1% (or even less).17
How many will eventually become infected (and how quickly) depends on how we respond. If spread is uncontrolled, models suggest that at the start of the epidemic each infected person will infect on average another ~2-6 people (and perhaps more).18 Interventions like social distancing reduce this number substantially. If each person infects between 2 and 6 others, it’s been estimated that ~50-80% of the population would need to develop immunity (most likely through being infected and surviving) in order for the population as a whole to have “herd immunity.”19. If the virus spreads uncontrolled worldwide, this would likely result in tens of millions of deaths.
However, many governments are now acting to suppress the spread. The best all-things-considered forecasts we know for total eventual reported cases and mortality are from the Good Judgement Project. For example, as of 3 April, forecasters think there’s a 24% chance that more than 8 million people will die worldwide from COVID-19 before 31 March, 2021 and a 17% chance that less than 800,000 will.
Getting plenty of sleep and exercise can help strengthen your immune system.21 We’ve also heard some evidence that Vitamin D reduces the frequency or severity of respiratory infections, though this is a less common recommendation.
Expert consensus seems to be that wearing a surgical mask does help prevent you from infecting others. Due to the shortage among healthcare workers, healthy members of the public shouldn’t buy surgical masks (or N-95s) at the moment. For this reason, we opted not to review the evidence for apparently healthy people wearing surgical masks in ordinary life, though our impression is that they do offer some measure of protection.21
Homemade masks have not been rigorously tested, so we have to rely on weaker forms of evidence. The jury is still out on whether homemade masks are overall helpful, but there’s a common-sense case in their favour, and their use in some circumstances has recently been endorsed by the US CDC.22
There is currently no vaccine or other proven pharmaceuticals. The main treatments provided by hospitals are things like life support in severe cases (e.g. assistance breathing) so patients live long enough for their immune systems to eliminate the virus.
Blood plasma from recovered COVID-19 patients has been used with some success in treating critically ill cases. The treatment has not been thoroughly tested or approved, but results from early cases of “compassionate use” seem promising.23 Unfortunately, scaling up this treatment is a challenge because it requires blood from donors.
There are many trials of antivirals being undertaken, which could reduce severity. It’s very hard to predict, but it seems plausible we could see some antivirals developed within a year.24
Developing a vaccine will take at least a year, and potentially several years.25
According to investigators from the Wuhan Medical Treatment Expert Group for COVID-19 as reported here, 84% of COVID-19 patients experienced loss of appetite, 29% had diarrhea, 0.8% experienced vomiting, and 0.4% had abdominal pain. It’s hard to draw a conclusion here, because some of this contradicts earlier evidence from the 28 February paper cited in the previous footnote, which found diarrhea in only 2% of cases at admission.↩
The Chinese CDC found that 80.9% of cases reported in China up to 11 February were ‘mild’, meaning without pneumonia or with only mild pneumonia (so you can still feel quite sick with a mild case), 13.8% were ‘severe’, involving shortness of breath and lower blood oxygen levels, and 4.7% were ‘critical’, meaning they had respiratory failure, septic shock or organ failure. See Table 1 for the figures and ‘Variables’ for the definitions.↩
As reported by the Centers for Disease Control, testing on the Diamond Princess cruise ship found 46.5% of passengers who tested positive were asymptomatic at the time of testing. However, COVID-19 has a long incubation period, and these people may have become symptomatic later. A 26 March follow up suggested based on modeling that 17.9% of those who tested positive never developed symptoms.↩
Even if the worldwide IFR is relatively low right now, it would not be surprising if infection fatality rates were considerably higher in places where the medical system is overwhelmed, such as Italy, or in the future if the infection spreads to a greater proportion of the population and hospitals are overwhelmed in more places.↩
The joint mission report cited above reports a case fatality rate for the studied patients higher among men vs. women (4.7% vs. 2.8%), and a 21 February report from the Chinese CDC found a case fatality rate for men of 2.8% v.s. 1.7% for women. It’s not clear, however, whether this difference is attributable to sex or something else that correlates with sex.↩
It’s difficult to know how much transmission is due to mild and asymptomatic cases, as they usually go unrecognized. A 16 March paper “Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus”(SARS-CoV2)” tried to find how much of COVID-19’s spread was due to these unrecognized cases in China, and reported that “Per person, the transmission rate of undocumented infections was 55% of documented infections ([46%–62%]).” However, we don’t know how mild the symptoms were in these undocumented infections. In general, there is controversy about the relationship between symptoms and infectiousness.↩
Up To Date’s guidelines on home care cite the CDC and WHO recommendations. There is some reason to think infectiousness peaks at the beginning of symptoms (see footnote 14), though even if that’s right we don’t know how long it takes to subside after that.↩
According to data collected by Our World in Data, by 14 March the US (population 327 million) had done 37,646 tests; Japan (127 million) had done 14,901 by 19 March, and India (1.3 billion) had done 14,514 by 20 March. On the other hand, by 20 March South Korea (51 million) had done 316,664 tests and Taiwan (24 million) had done 21,376. This suggests large variation in the number of cases being captured in each country.↩
“Herd immunity” is a phenomenon whereby a large enough portion of a population is immune to a virus, whether because they have already been infected or from vaccination, that the virus is mostly unable to spread in the population, even to those not immune. The threshold of infection for a population to have herd immunity depends on how many people the average infected person infects (the ‘R0’), and how common and long lasting immunity is (which is unknown for those who have been infected with COVID-19, but thought to be reasonably common and long-lasting, as with other coronaviruses), among other things. The 50-80% figure comes from the 7 April “early release” cited in the previous footnote, according to which for an R0 of 2.2 the threshold for herd immunity is 55%, and for and R0 of 5.7 it is 82%” The threshold for herd immunity could be even higher if, as the authors of that paper say might have been the case in the early days of the epidemic in Wuhan, the R0 is even higher than 5.7. The UK’s chief scientific advisor Patrick Vallance gave a figure of 60% for the UK in mid-march, as reported by Sky News and repeated in the Financial Times.↩
This is the main advice we’ve seen from many sources — though different institutions have slightly different recommendations. See e.g., guidelines from the NHS, the CDC, and the Hong Kong government.↩
Some worry that homemade face masks could cause or worsen infections by capturing viruses and bringing them closer to the face, or if they are shared or not washed. They might also give people a false sense of security, causing people to be more cavalier about going out and being near other people, and even proponents agree they are not highly effective at preventing infection, either for the wearer or those around them. All that said, they may be better than nothing. This post makes the case for using homemade masks to help decrease transmission.↩
On 3 April the FDA announced plans to develop blood therapies, including convalescent plasma, for COVID-19, while allowing their simultaneous use for critically ill cases without other options. The sense of promise for these therapies is based on past successes treating other respiratory viruses with these methods as well as preliminary reports of improvement in COVID-19 patients given plasma (1, 2). These reports are not from controlled trials, which have not been conducted, and are based on use in 10 and 5 cases, respectively.↩