University of Cambridge researchers explain why Covid-19 mortality has fallen in the second wave
Wearing masks, social distancing and good ventilation have played an important part in reducing the mortality rate from Covid-19 during the second wave, according to Cambridge researchers.
They say the severity of the infection is related to the amount of virus an individual is exposed to, meaning that separation measures not only reduce the risk of transmission but also cut your chances of suffering from severe disease.
The number of Covid-19 cases seen in many European countries rose sharply as the second wave of the coronavirus hit, but it was not accompanied by such a steep rise in hospital admissions or deaths.
Researchers from the University of Cambridge’s MRC Biostatistics Unit and Department of Medicine, together with Imperial College London and St George’s, University of London, have proposed three main reasons for the decline in rates of severe illness and mortality as the pandemic has progressed.
The first is that the age demographic of cases has changed, with proportionally fewer infections being seen in over-60s. This could reflect greater awareness of the heightened risk for this age group and the measures taken to protect them.
In their editorial for the BMJ, the researchers write: “In many Western countries, the absolute number of cases in people older than 60 is increasing but the proportion of cases in that age group is decreasing. This indicates that public health measures to curb the spread of SARS-CoV-2 have been successful in reducing the relative burden of disease in older people, who are the most vulnerable demographic group.”
The second primary reason for the decline in severe illness and mortality is the improvement in treatment. Thanks to randomised trials, we now have drugs that can help treat some severe cases of Covid-19 and clinical management of patients has improved as medics have acquired greater experience of it.
The third reason proposed is that individuals may on average now be taking in a lower amount of the virus at the point of infection, thanks to those separation measures - keeping two metres apart, wearing masks and increasing air changes by opening windows.
The researchers write: “Although human challenge trials for SARS-CoV-2 are in early stages of development, studies of influenza A virus in humans have shown that initial exposure to a lower inoculum results in fewer and shorter symptoms as well as lower likelihood of viral shedding.”
They add: “Viral load in patients with Covid-19 has been shown to be greater in those with more severe illness.”
Studies using hamsters and ferrets have shown reduced rates of severe Covid-19 disease from those exposed to lower viral doses.
Masks have also been shown to be effective.
“In humans, randomised trials of patients with viral upper respiratory tract infections have shown mask wearing reduces viral droplet emissions. Furthermore, masks have been found to reduce inhalation of particles,” the researchers note..
Several observational studies have linked the use of masks with lower rates of severe Covid-19 outcomes in humans.
And a study involving hamsters in laboratory conditions also suggested the value of masks. In this case, surgical masks put up across partitions between groups of hamsters resulted in lower rates of disease transmission and less severe outcomes, compared to hamster groups in which the air circulated freely between the partitions.
Keeping apart has also been shown to be effective.
“A case series in humans comparing three transmission clusters with different degrees of social distancing showed differences in outcome severity between the clusters,” the researchers write.
The evidence presented supports public health messages surrounding separation measures.
Stephen Burgess, group leader at the Medical Research Council Biostatistics Unit, University of Cambridge said: “Wearing a mask and only meeting in well-ventilated areas will not only reduce the risk of catching coronavirus, but also reduce the chances of developing severe disease if you do get infected.”
Dipender Gill, a clinician scientist at Imperial College London and St George’s, University of London, added: “The available evidence supports the implementation of public health measures that are most effective for reducing levels of viral exposure. This will help create a balance between controlling the Covid-19 pandemic while allowing society to continue to operate.”
Case fatality rate - the ratio of confirmed Covid-19 cases to deaths - was at its height in the UK in April, reaching 15.1 per cent. On December 12 in the UK, it was 3.5 per cent, according to data from John Hopkins University.
It is important to note, however, that this is not the same as the risk of death from Covid-19, as it only reflects confirmed cases. The figure will also be impacted by the scaling up of testing efforts in the UK.
Sign up for our weekly newsletter for a digest of the latest Cambridge science direct to your inbox