Public anxiety has been heightened by recent reports of the new British, South African and
Brazilian strains of the virus that causes Covid-19.
The virus, SARS-CoV-2, has RNA rather than DNA as its genetic basis. Viral replication requires the transcription of copies of its genome, and RNA is inherently less stable than DNA, so it is a feature of RNA viruses that they are likely to have copying errors that lead to random mutations.
Most of these are probably fatal to the virus, and some are neutral, but inevitably there will be some that produce a transmissable virus with properties that differ from the parent strain. If the new variant is better at getting itself into a new host, making copies in them, and spreading itself to further hosts, then over time it will overtake its parent strain and become dominant. That is what seems to have been occurring with these new strains.
The rate at which a virus transmits itself through a population is described by the reproduction number R, now familiar to most of us as a result of news coverage.
It describes the average number of new cases that are produced by each sufferer from the infection. If it is less than 1, then each case produces fewer than one replacement on average, and the epidemic will fizzle out. If it is greater than 1, then the number of infected subjects will increase, and the disease will spread. R is determined by the infectivity of the virus and by the contacts that an infected person has with susceptible people. The number and nature of contacts between people are societal variables, so
values cannot necessarily be generalised from one place, population or situation to another.
The evidence we have is that for the original strain of SARS-CoV-2 in an unimmunised population like ours without specific social distancing, R0 is about 2.7: that is, each case will infect between 2 and 3 people to produce new cases. Of course this is a statistical average; some individuals will infect no others and some "superspreaders" may infect many.
If we take 3 as a rounded number, then one infected individual will on average produce 3 more, each of whom will produce 3 more.
The generation time for SARS-CoV-2 is about 5 days, so after three generations (15 days) we expect 27 infections, in a further 5 days 81, then 243... This is the "exponential spread" that leads to widespread disease.
So how much more infectious are the UK, SA and Brazilian strains of SARS-CoV-2? The
published evidence is that their infectivity is "about 30 to 70 percent higher" than the preceding dominant strains. That would correspond to an increase in R0 from about 2.7 to between 3.5 and 4.6, a change that gives a markedly increased rate of unconstrained spread. If instead of 3 we take 4 as the number of new cases generated from each case, then instead of 3-9-27-81 - 243 in successive generations of infection we would see 4 - 16-64-256-1024.
It is easy to see that this is a significant worsening of the public health problem, but the risk from an individual contact is not enormously greater, so it is not something we need to individually find terrifying.
In comparison, R0 for measles in an unimmunised population is about 12 to 18.
To control an epidemic we need to reduce R below 1, and that can be achieved by limiting the number of contacts, by limiting the nature of contact, or by making sure that those with whom an infected person has contact are immune to the disease.
The strategies used are social isolation or lockdown to reduce the number of contacts; physical distancing, handwashing and masking to limit the nature of contacts; and immunisation to reduce the number of susceptible contacts.
For the new, more infectious, variants the application of these strategies needs to be more assiduous to achieve the desired lowering of R, but there is no doubt that they can work.
Another feature of the UK strain is that it appears to have higher mortality than the preceding dominant strains. We are told it is "30 per cent more lethal". Again, this should be considered in perspective.
The previous UK case fatality rate for Covid-19 in 60-year-olds is estimated at about 10 per 1000, and for the new strain it appears to be about 13 per 1000. That translates by simple maths to a probability of surviving an attack of 99 per cent for the previous dominant strain, and 98.7 per cent for the new one. This is not the stuff of which nightmares are made.
One of the potential problems with these new strains is that the immune response produced by vaccines may be less effective against them. The vaccines are mostly engineered to produce immunity directed against the virus spike protein. That region is constrained from changing dramatically because it is the part of the virus that docks with human cells to cause infection, but all of these three have minor variations in it.
Laboratory testing suggests that immunity from the Moderna vaccine to the UK variant is unchanged, but that it has reduced efficacy for the SA variant.
That may or may not translate into reduced protection in real life.
The good news for New Zealand is that the Pfizer/BioNtech vaccine that the first to be delivered for us to use appears to provide good protection against the UK and SA variants.
The AstraZeneca/Oxford vaccine confers protection against the UK variant similar to its protection against the previous dominant strain, although its effectiveness against the SA strain is reduced.
It is inevitable that as the proportion of immunised individuals in the population increases, variants will arise that escape vaccine-induced immunity to some extent. The strategy used to produce these vaccines, by laboratory synthesis of nucleic acid sequences that code for viral proteins, should readily allow adjustment of those sequences to follow changes in the virus over months and years.
Covid-19 is a nasty disease. While its severity and case fatality rate for young and healthy people are generally low, it causes significant ongoing health problems for some. In older people, and in those with conditions such as respiratory disease, diabetes and morbid obesity, it may cause very severe illness and is sometimes fatal.
It is certainly something we should take seriously and do our best to avoid and to contain - but let's keep it in perspective.
• Ross Boswell is a pathologist and physician in the public hospital system.