the science of Covid-19: possible treatments, herd immunity
Australia is now 22nd in the list of total COVID-19 cases and dropping down. There are still of course new cases every day, but far fewer than many countries below us on the list. Experts are now talking about a flattening curve, but they also warn that the virus is ‘here to stay’. Here in South Australia, there were no fatalities when I last posted, but there have been three in the last couple of days. There are a large number of cases recently linked to our Barossa wine-growing region, a popular tourist destination.
So let me take a closer look at the SARS-CoV-2 virus. It’s a positive-sense single-stranded RNA virus. RNA is generally single-stranded in nature, though apparently can be double-stranded on occasion. The positive-sense term refers to the polarity, or sense, of the RNA. It’s also called ‘positive-strand’, facing 5’ to 3’, which means it acts as mRNA and can be translated into viral proteins in the host cell.
These types of virus are very common. They include common cold rhinoviruses as well as the SARS and MERS coronoviruses. SARS-CoV-2 is genetically similar to bat coronaviruses, causing virologists to believe that it was transmitted from bats to humans through an intermediate species such as a pangolin. The reproduction number of the virus (R0) is currently ranged from 1.4 to 3.9, in a scenario of no immunity and no preventive measures taken.
It has often been repeated that a vaccine will take 12-18 months, if not longer, to be safe, ready and effective. Science communicators such as the ABC’s Dr Norman Swann are telling us that stay-at-home orders may need to stay in place until that time, which is surely alarming economists and the business community. So, unsurprisingly, people are looking to short-cuts and desperate remedies. Perhaps the most publicised of these is the anti-malarial drug hydroxychloroquine, aggressively promoted by the US President. It turns out, also not surprisingly, that he has some financial interest in the French company that has branded the drug, according to the New York Times. There doesn’t appear to be any clear evidence on the benefits of the drug. Best reports speak of ‘mixed results’.
There are reports also of the benefits of blood plasma from people who have recovered from Covid-19. A small Chinese study involved 10 severely affected patients being given a few hundred millilitres of ‘convalescent plasma’ containing viral antibodies, and results were described as promising. The approach is being tried in the US, with the Red Cross and the American Association of Blood Banks seeking to recruit suitable ‘fully recovered’ donors.
As people continue to be alarmed and frustrated at the massive disruption to their working and social lives caused by Covid-19, some world leaders (e.g Boris Johnson and his chief science adviser Patrick Vallance, and Netherlands PM Mark Rutte) have come up with not-so-encouraging solutions, such as allowing the virus to run its course so that the population can build up herd immunity. This would actually be a disastrous policy in the case of a virus with a high (but not precisely known) fatality rate, involving millions of severe cases requiring intensive care treatment at any one time.
Herd immunity occurs when enough people have antibodies to the virus that it has nowhere to go. This can occur through the work of our immune systems or through antibodies created by effective vaccination. The former obviously comes at a much greater cost in terms of lives lost, in the case of a highly infectious (the R0 is now estimated – the data changes as I write – at between 2.0 and 2.5), high-fatality virus. Also, because Covid-19 is new, we don’t have sufficient data as yet about the degree of immunity it confers upon recovered patients, or whether it is able to mutate to any degree. Experts are generally counting on low or no mutation, but none of them see relying on herd immunity to be a humane solution to the problem. Suppression is the name of the game at the moment (even though it will reduce herd immunity). That’s to say, the R0 mentioned above (which might be higher) is the figure without the application of physical distancing or other containment measures. The R0 number, if it can be ascertained, gives an indication of the percentage of immunity required to ‘protect the herd’. An R0 number of 2 will require about 50% immunity. If the R0 number is 3, some 66% immunity will be required. Measles has a very high R0 of 12, requiring 90% immunity, which explains why anti-vaccination movements can imperil whole communities.
So it’s a trade-off. Physical distancing measures will reduce the possibility of herd immunity – the production of antibodies. Going back to business as usual will increase infection rates – ok for those who recover, not so much for those who don’t. The cost of the second option, most will agree, is just too great.
References
Leave a Reply