Covid-19 spreads quickly, but the scientific community is working at an equally remarkable pace to formulate, trial, manufacture and distribute vaccines; develop more rapid, reliable and less invasive testing; and understand exactly how the novel coronavirus develops in communities, as more variants emerge in different settings.
The speed of data-gathering and New Zealand's changeable alert levels makes this the optimal time to reflect on what we have lately learned about Covid:
Symptoms likely correlate with transmission. According to the British Medical Journal, symptomatic and pre-symptomatic transmission (beginning 1-2 days before the onset of symptoms) are likely to play a greater role in spreading the virus than asymptomatic transmission. That is, people who carry the Covid-19 virus (SARS-CoV-2) but never develop symptoms (i.e. remain "asymptomatically infected" until clearing the virus) are much less likely to transmit virus infection compared to those who have symptoms, or are about to develop symptoms.
Detection of the Covid virus doesn't necessarily tell us much about infectiousness. Part of what we still need to understand about Covid disease is for how long the average case is infectious for. PCR tests can detect Covid RNA for a mean of 17 days (and maximum of 83 days), but the detection of viral RNA does not necessarily equate to infectiousness. This is why we occasionally see community cases in New Zealand where people have tested positive but the authorities aren't worried about community spread. This is because it is a historical case where RNA remains detectable but the person has not been infectious for some time.
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In most cases, the viral load (numbers) is replaced by antibody response. After the initial exposure to the virus, people who contract the virus disease normally develop symptoms within 5-6 days (incubation period). In people with mild infection, the majority of symptomatic cases, our initial immune response is capable of controlling the infection early and well. The viral load peaks in the first week of infection and then declines gradually, while the antibody response then gradually increases and is often detectable by day 14 after first exposure.
We need both antigen and antibody tests for Covid. Antigens and antibodies, immune proteins detectable in our blood which are created by our immune system to neutralise the virus, are considerably more stable than RNA, which makes them less susceptible to spoliation during transport and storage of tests, therefore reducing the chance of false-negative results. Testing accuracy is also improved by the fact that antigens and antibodies are more uniformly available in saliva and blood samples. The biggest advantage of antigen/antibody tests is their ability to detect past infections.
However, antibody tests have their limitations. As immunological data continues to emerge, it is becoming apparent that the body's antibody response to Covid infection is considerably slower than we might have expected. While data at this point is still limited, it appears that the initial IgM (Immunoglobulin M) antibody response doesn't peak until at least approximately nine days after initial infection and the IgG (Immunoglobulin G) antibody response doesn't peak until approximately day 11.
We still have much to learn about the finer points of transmission. We know that most super-spreading events have occurred indoors, and that infection risk increases substantially in enclosed environments compared with outdoor settings. As most Kiwis are now aware, most transmission occurs through close-range contact, e.g. 15 minutes face-to-face and within two metres, meaning Covid virus spreads especially efficiently within households and through gatherings of family and friends. Household secondary attack rates (the proportion of susceptible individuals who become infected within a group of susceptible contacts with a primary case present) range from 4-35 per cent.
However, the maximum capacity for transmission after the first week of illness has not yet been well-documented. Severely ill or immune-compromised patients may have relatively prolonged virus shedding and some may have intermittent RNA shedding, while asymptomatic people (those with no symptoms throughout their Covid infection) seem to have relatively limited infectiousness. People with mild symptoms and the pre-symptomatic (starting one to two days before symptom onset) carry large amounts of virus in the upper respiratory tract, which might contribute to easy and rapid spread.
Forty to 60 per cent of all those who catch and carry SARS-CoV-2 virus, so are "infected', never develop symptoms (remain asymptomatic) so do not develop Covid disease. Some of these people could transmit the virus to others but are generally thought to be much less infectious to others.
• Ben Harris is a medical microbiologist who has spent more than 40 years working in the UK and across New Zealand. He has advised and lectured at numerous hospitals, universities and labs as well as serving on multiple government infectious diseases committees.