Is SARS-CoV-2 Virus Weakening?

A recent study in Hong Kong provides an answer.

On December 16, 1904, a bacteriologist named Theobald Smith proposed his “law of declining virulence.” Smith argued that natural selection would predict that pathogens should become more contagious and less virulent over time.  Why, he asked, would a pathogen evolve to kill its host when it results in the death of both.

To Theobald Smith, rhinoviruses, which cause the common cold, were the perfect pathogen. Rhinoviruses are highly contagious, and virtually never fatal. The virus wins; the person wins. Many viruses, however, haven’t followed Smith’s treatise. For example, rabies is not particularly contagious (spread only by the bite of an infected animal) and invariably fatal (once symptoms begin, rabies is 100 percent fatal); it’s the deadliest pathogen of humankind. Rabies virus was first described about 4,000 years ago, so it’s had plenty of time to become less virulent. Similarly, influenza virus, which has been around since the mid-1300s, caused a uniquely fatal worldwide pandemic in 1918, killing about 50 million people. Smallpox virus, which was first described about 3,000 years ago, is estimated to have killed about 500 million people before it was eliminated by a vaccine; it, too, never weakened.

What about SARS-CoV-2 virus? SARS-CoV-2 first appeared in Wuhan, China, in late 2019. The original strain, called the ancestral strain, evolved to create a series of variants that were more contagious but no less deadly, such as the D614G, alpha, and delta variants. Then, in late 2021, the omicron variant raised its head. An initial report out of southern Africa found that while omicron was causing a dramatic increase in cases, it wasn’t causing a dramatic increase in hospitalizations and deaths. Was omicron weakening—fulfilling Theobald Smith’s prediction? One early study, titled, “Omicron—The Latest, the Least Virulent, But Probably Not the Last Variant of Concern of SARS-CoV-2,” claimed that it had. Indeed, one prominent public health official was so excited about the possibility that SARS-CoV-2 virus was finally weakening, he declared, “This is possibly the live, attenuated vaccine we have all been waiting for.” The hope was that people infected with omicron would develop protective immunity without having to pay the high price of natural infection that had been exacted by previous variants.

During the early days of omicron, an opinion piece in the New England Journal of Medicine sounded a more cautious note. The authors pointed out that population immunity was much greater later in the pandemic than early in the pandemic. By the time omicron was circulating, most people had already been either immunized, naturally infected, or both. Maybe this was why omicron had appeared to be less dangerous.  The only way to answer this question would be to study people who had not been vaccinated or naturally infected early in the pandemic, before omicron, and compare them to people who had also not been vaccinated or previously infected later in the pandemic, during omicron.

On June 27, 2023, researchers in Hong Kong did just that by studying six epidemic waves of Covid-19 from January 2020 through October 2022. The first four waves were pre-omicron, the last two were during omicron. Researchers found that before omicron, the hospitalization rate for those over 80 was 39%; for those 65-79, it was 11 percent. During the two omicron waves, the hospitalization rate for those over 80 was also 39%; for those 65-79, it was 19 percent. According to this study, omicron hadn’t weakened.

To date, SARS-CoV-2 virus doesn’t appear to be on its way to becoming just another common cold virus. For the foreseeable future, the virus will continue to cause serious harm in those who are most vulnerable.