A new scientific study made international headlines last week when it proved that Covid-19 had been circulating in the USA at least a month earlier than had previously been demonstrated. Blood donations collected in nine states by the American Red Cross between 13 December 2019 and 17 January 2020 were tested. Blood from all nine states – California, Connecticut, Iowa, Massachusetts, Michigan, Oregon, Rhode Island, Washington, and Wisconsin – tested positive for anti-SARS-CoV-2-reactive antibodies.

The earliest studied samples were from the west coast (California and Washington state), collected 13-16 December. Of these, 2 per cent of these individuals had been infected with Covid-19 and had time to develop antibodies. It takes around three weeks after infection for the body to develop sufficient antibodies to be identified by a test.

We do not know the health or demographic profiles of the infected individuals, although summary data is included in the study. The median age of the donor was 52 and more men donated than women. Ethnicity was not reported, although earlier studies suggest that people in their forties donate most of all (27 per cent of blood donated) and that the vast majority of blood donors in the USA are white (78 per cent), who donate at about twice the rate of black Americans, and almost four times the rate of Hispanics.

None of these donors was feeling ill. Red Cross guidelines require that donors be “in good health and feeling well”. Those experiencing any cold or flu-like symptoms are instructed to cancel an appointment, and all donors must be sixteen or older.

Those who have been exposed to multiple human coronaviruses are less likely to contract Covid-19

The possibility of false positives was addressed in the study: all positive results were subjected to more sensitive testing to distinguish with greater accuracy SARS-CoV-2 antibodies from those produced following infection by other coronaviruses. The presence of other such antibodies in the blood is the subject of equally fascinating, and potentially more rewarding research. It is interesting to know that Covid-19 has been around longer than we thought, but we already knew that the vast majority of cases are asymptomatic or produce mild to moderate symptoms. However, it is of far greater note that pre-existing antibodies may offer some immunity and could be a reason why so few adults, and almost no child, contracts severe Covid-19.

A study published in Science suggests that “immune cross-reactivity among seasonally spreading human coronaviruses” is real, such that those who have been exposed to multiple human coronaviruses have greater immunity to the novel coronavirus, are less likely to transmit the virus, and are less likely to contract Covid-19. It concludes that this may be a partial explanation for why children are almost all asymptomatic and unlikely to spread Covid-19.

Other than existing antibodies present in the blood, scientists have been looking closely at whether blood type affects severity of infection. A Harvard/Mass General study published in July concluded that there is no obvious correlation between blood type and the severity of Covid-19 infection. However, it supported earlier finding that “patients with blood types B and AB who received a test were more likely to test positive as were those who are Rh+ positive, and blood type O was less likely to test positive”.

A study published in Nature, based on 14,112 individuals tested within the New York Presbyterian hospital system, has since observed that “the prevalence of initial infection was higher among A and B blood types and lower among AB types, compared with type O”. The verdict on the ABs, therefore, is disputed. But it seems certain that is good to be an O, better to be an A, and best of all to be Rh- negative.

Close monitoring of wastewater is an essential tool for public health officials

Stool is more important as blood in understanding the spread of Covid. Although it remains unlikely that “toilet plume” is to blame for many infections, wastewater-based epidemiology (WBE) has come of age in the Covid era. It is now clear that SARS-Cov-2 RNA virus is shed in faeces even by those who are pre-symptomatic or asymptomatic, and who therefore are unlikely to be captured by clinical screening. In Spain, a study conducted in March and April 2020 in Murcia, the area that then had the lowest Covid-19 prevalence in Iberia, demonstrated that people “were shedding SARS-CoV-2 RNA in their stool even before the first cases were reported by local or national authorities in many of the cities where wastewaters have been sampled”.

Several published studies have been retrospective, for example identifying SARS-Cov-2 in Italian wastewaters from Milan and Rome that correlate with the earliest identification of clinical cases of Covid-19. Like testing of blood donations, WBE studies have the potential to push back the earliest date at which the virus was circulating considerably. More useful is a study of Louisiana sewage plants that quantifies “the percent recovery of SARS-CoV-2 RNA from wastewater”.

Since fluctuating levels of SARS-CoV-2 in sewage are an excellent indicator of peaks and troughs in infection, close monitoring of wastewater is an essential tool for public health officials. Not only is “WBE surveillance of populations is … orders of magnitude cheaper and faster than clinical screening”, but it also captures a different, broader picture, not a selection of details in high-resolution that grab headlines. What governments do with the information extracted from sewage plants is another matter.