Covid-19 has wreaked havoc across the globe since it first appeared a year ago. The virus has killed more than a million, infected many millions more, damaged economies and put the world in retreat. News of the success of several vaccines including Pfizer and Moderna's brought hope the pandemic may be ending. But there is still much we don't know about the deadly virus. Here are five:
How long immunity will last
People who have been infected with the coronavirus typically produce antibodies, which are created by types of white blood cells called B cells. Other white blood cells called T cells are also key to the immune response.
Studies have shown antibody count in the blood may fall significantly after a person has cleared a coronavirus infection.
Despite this, the person may still be resistant to a further infection, because the immune system retains a “memory” of the original infection and so is able to mount a renewed response quickly. It is not known how long this immunity lasts.
It is also unclear how long immunity generated by a vaccine, which may differ in longevity from immunity after infection, will persist.
These unknowns about immunity from vaccination and infection can only be resolved, for obvious reasons, with the passage of time.
If coronavirus vaccines prevent transmission as well as illness
While several coronavirus vaccines have been shown in clinical trials to be highly effective at preventing illness, what is less clear is whether they can also stop transmission of the virus. This is key to cutting the virus’s prevalence.
There are positive signs with the University of Oxford and AstraZeneca vaccine. The university said there is an “early indication that [the] vaccine could reduce virus transmission” because fewer asymptomatic infections were seen among those who had been given the jab.
The World Health Organisation’s chief scientist, Dr Soumya Swaminathan, has said it is thought “it needs at least 60 to 70 per cent of the population to have immunity to really break the chain of transmission”.
However, some surveys show that a significant proportion of the population in some countries will not get themselves vaccinated.
A US survey in September found just 51 per cent of people there were willing to be vaccinated, with concerns centred on possible side effects and the effectiveness of vaccines, a potential stumbling block to efforts to create “herd immunity”, which cuts the spread of infectious diseases.
How long lasting the symptoms of infections will be
While many people with the coronavirus get back to normal quickly, or do not have any symptoms in the first place, others suffer effects that are long lasting.
Just how long that “long covid” can last remains to be seen, although experience with the Severe Acute Respiratory Syndrome (SARS) epidemic of 2002 to 2004 in Asia suggests that the effects in some patients may extend for a number of years.
One study of SARS patients found that the lung function of more than half of individuals was still affected two years after their illness.
With the new coronavirus, effects have persisted even in some people whose initial symptoms were relatively mild.
There is a wide range of long-lasting symptoms encompassing breathing difficulties, fatigue, depression, headaches, dizziness, neurological problems and kidney damage.
High levels of inflammation can cause damage to the heart, potentially increasing the risk of a heart attack, with research in Germany suggesting that even younger people may be affected longer term.
Why some people suffer much more severe symptoms than others
It is well known that certain groups are much more vulnerable to the coronavirus than others. Notably, elderly people, whose immune systems are likely to be weaker, along with men, people who are overweight, members of some ethnic groups and individuals with serious medical conditions have suffered much higher death rates.
However, sometimes younger people without obvious risk factors have become severely ill and even died, and clinicians have said that it is difficult to know in advance who will fall into this category.
Some individuals have weak production of proteins called type I interferons and excessive production of substances called cytokines, a pattern associated with a potentially fatal state of hyperinflammation.
Genetic factors have been linked to some cases of severe illness among younger healthy people and members of other groups.
However, much remains to be understood about why some healthy individuals can deal with the virus without any illness, while in others the immune system ends up attacking the body’s own tissues and severe illness results.
How many people have actually been infected with the coronavirus
According to official World Health Organisation figures, there have been more than 63m confirmed cases of Covid-19, but the real number of infections, which is much higher, is unknown.
In early October, Dr Michael Ryan, the WHO’s head of emergencies, estimated that about one in 10 people worldwide had been infected, which translates to about 760 million cases. The official number of positive cases at the time was just over 35m or less than five percent the estimated actual total.
Studies have produced estimates of actual case numbers by testing for antibodies produced in response to the coronavirus.
For example, a study by scientists at Imperial College London, based on antibody testing of tens of thousands of people, suggested that by mid-July about 3.4 million people in the UK had had the disease. At that time the official number was below 300,000.
The ratio of official cases to actual cases will differ significantly from country to country because testing capacity varies.