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How do antivirals affect the risks we face and the way we behave?

The recent announcement that antiviral treatments for Covid-19 will soon be available may reduce people’s incentives to avoid becoming infected and potentially lessen their readiness to practice social distancing or indeed to get vaccinated.

More than at any other time in living memory, the experience of Covid-19 has shown the frightening power of infectious diseases to wreak havoc on everyday life, radically changing how we interact, work and trade. The lack of effective pharmaceutical products during the early stages of the pandemic meant that we had to resort to very costly behavioural interventions to break chains of transmission. Social distancing and lockdowns cut people off from loved ones, and curbed our ability to work and learn.

Since the rollout of vaccines got underway in early 2021, the debate has shifted away from lockdowns and social distancing, and towards vaccine hesitancy, vaccine mandates and Covid-19 immunity certification. Yet the announcement of two effective antiviral drugs marks a potential new milestone in the effort to manage the pandemic.

It may be difficult to remember now, but there was a time in 2020 when vaccines seemed a distant prospect and the smart money was on the development of antivirals – for example, through the repurposing of existing drugs. Vaccines have now become a cornerstone of public health measures to control the spread of the disease.

Yet antivirals can also play a crucial role in public health policy, adding an important tool to options that are currently available. Recent announcements of the approval of two antiviral drugs – molnupiravir from Merck and ritonavir from Pfizer – are welcome news. But the introduction of antivirals also raises a number of important questions about people’s incentives and behaviour.

How do antivirals affect behaviour?

Just as the introduction of vaccines may alter behaviour, so too can the introduction of antiviral medications. It is useful to contrast the effects of vaccines and antivirals. Vaccines have three main properties:

  • First, they may reduce the probability that an exposed person becomes infected.
  • Second, they may reduce the severity of illness in those who do become infected.
  • Third, they may reduce the probability of onward transmission to others.

The extent to which a vaccine has these properties determines how useful it is as a public health measure and how much people’s individual incentives to get vaccinated contribute to desirable outcomes for society as a whole.

Antiviral drugs can also have several effects:

How do these properties influence incentives and behaviour? What are the trade-offs that people face and do antivirals change the choices they make? As with the economic analysis of vaccines, to answer these questions it is useful to separate cases in which antivirals interact with behaviour and when they do not.

How do antivirals affect public health?

From a public health perspective, antivirals can be thought of as a tool to hasten the build-up of herd immunity. In the same way that vaccines induce increased immunity in susceptible people, so antivirals may shorten the time that people are infected and thus speed up natural immunity from recovery.

For that reason, mass rollout of antivirals can be a valuable tool of public health as they can alter the course of the disease in the population (the aggregate disease dynamics). Studies show that the public health value of antivirals is largest in the early stages of an epidemic, when there are still many susceptible people at risk of infection (Toxvaerd and Rowthorn, 2020).

How do antivirals affect individuals’ decisions?

Turning to behaviour, it is useful to distinguish between people’s incentives before and after they have become infected. Once infected, the choice facing an individual is a simple one, namely whether or not to get treatment. What a person decides will depend on the cost (and possible side-effects) of treatment and on the expected severity of the disease. Otherwise healthy individuals may choose to ride out the infection, while more at-risk and vulnerable people may choose to get treated.

Importantly, and in contrast with the choice of whether to get vaccinated, the decision to seek treatment is wholly disconnected from the state of the epidemic and the treatment decisions of others in the population.

To see this contrast clearly, consider an individual’s decision on whether to get vaccinated. For starters, this person may factor in the overall risks of getting infected, which of course will depend on the state of the epidemic and especially on how many infected people to whom they are likely to be exposed.

In addition, because of the build-up of herd immunity, the decision may well be conditioned on others’ decisions to get vaccinated themselves. As more people get vaccinated, the less urgent it may seem for any individual to get vaccinated because they benefit indirectly from the vaccination of others. Thus, there is an element of ‘free-riding’ in vaccination decisions.

This is not the case with the decision to seek treatment. An infected individual does not benefit more or less from treatment when other infected individuals seek treatment, as long as there is significant natural immunity. In cases where there is not, the analysis of infection dynamics with treatment becomes significantly more involved (Rowthorn and Toxvaerd, 2020).

Because the decision to seek treatment is unaffected by the stage of the epidemic (for individuals) or by what other people do, there is a greatly reduced scope for ‘game playing’ or strategic behaviour where people try to predict or second-guess the decisions of others and benefit from their actions.

In addition, because actual infection is more tangible than the risk of infection, the decision to seek treatment may be less fraught than the decision to get vaccinated.

Let us now turn to how the introduction of antivirals may change behaviour of those not already infected. In the face of serious infectious diseases like Covid-19, people may have strong incentives to protect themselves (see, for example, Toxvaerd, 2020). These incentives increase with the perceived severity of falling ill.

But one of the main effects of the new antivirals is that they significantly reduce the probability of hospitalisation and death among those who do become infected. This means that the availability of antiviral treatment may reduce the incentives to avoid becoming infected in the first place and can thus reduce people’s readiness to practice social distancing or indeed to get vaccinated (see Makris and Toxvaerd, 2020 for an analysis of pre-treatment social distancing incentives). In principle, the introduction of treatments could inadvertently help to spread the disease, rather than to contain it.

That a policy to contain infection may backfire and cause more widespread infection has been noted before and is known as ‘disinhibition’. It is a widespread phenomenon, but relevant in the disease context for policies such as face-mask mandates and pre-exposure prophylaxis against HIV/AIDS (see here for a non-technical introduction to such issues).

The possibility of disinhibition in response to antiviral treatments is one that is currently being taken very seriously by public health practitioners. Amesh Adalja of the Johns Hopkins University Center for Health Security recently wrote that:

'… it is natural to wonder if having these highly effective oral drugs will diminish the value or role of COVID vaccines in our response. There is a real fear being voiced by public health practitioners that if highly effective treatments stand at the ready, people who have so far shunned the vaccine will likely never get vaccinated.’

Similarly, a recent piece in Reuters after the announcement of the new antivirals stated that:

Some disease experts fear the arrival of oral COVID-19 treatments may further impede vaccination campaigns. Preliminary results of a survey of 3,000 U.S. citizens by the City University of New York (CUNY) School of Public Health suggest the drugs could "hamper the effort to get people vaccinated", said Scott Ratzan, an expert in health communication at CUNY, who led the research.’

Time will tell whether these fears materialise, but it is worth noting that such concerns appear to have been unfounded in other contexts. When emergency contraception (the ‘morning after’ pill) was first introduced, fears were expressed that women could respond by becoming less cautious, which could lead to more unwanted pregnancies. But a detailed empirical study of such effects suggests that availability of emergency contraceptives did not significantly alter behaviour. Whether this will also be true for Covid-19 is still to be determined.

Despite the potential behavioural effects that antivirals can create, the announcement of new drugs to lessen the effects of illness from Covid-19 should be welcomed. With people tiring of non-pharmaceutical interventions and the protection from vaccines diminishing over time, policy-makers need all the help they can get. If used wisely and with an eye on potential adverse behavioural effects, antivirals could become an important tool in the continuing battle against Covid-19. 

Where can I find out more?

Who are experts on this question?

  • Flavio Toxvaerd
  • Miltiadis Makris
  • Robert Rowthorn
Author: Flavio Toxvaerd
Picture by HeungSoon from Pixabay
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