Last week, the US hit its lowest number of daily new Covid cases since last fall, dropping to 53,800 from its high of 300,000 at the beginning of this year. After twelve months of watching cases steadily rise — twelve months that left many of us damaged, weary, and generally overwhelmed — many now cling to this new and encouraging trend as a sign of hope that the end of the pandemic could finally be near. Unfortunately, in my opinion, we are nowhere close to the end. But we may be approaching a welcome reprieve from our current tragedies and an opportunity to prepare for a much longer battle ahead.

Based on what we know of the virus that causes Covid-19, I believe we can expect the disease to return in waves each year. Long-lasting population or herd immunity like what we have for polio or the measles is not, in my opinion, a possibility for Covid-19, even with a vaccine. What we can expect instead is seasonal population immunity, with patterns of infection and reinfection that mirrors what we see with the flu. 

Influenza viruses and coronaviruses have many similar characteristics which should lead us to expect similar patterns of infection. Both viruses have proven able to evolve to escape our best defenses, be they vaccines, drugs or our own natural immune protection. This ability to shift shape is partly why influenza comes back each fall and winter in the Northern and Southern hemispheres to reinfect us. The dominant flu strain from one year is different from the flu strains that dominate in subsequent years, having accumulated small genetic changes as the virus passes through individuals, within communities, and in and out of other populations, like animals. Any immunity generated in response to a previous strain has only a limited effect, if any, in preventing infection from the new strain.

Based on what we know of SARS-CoV-2 and the cold-causing coronaviruses which have long circulated in our communities, coronaviruses can do the same. The first time we noticed SARS-CoV-2’s ability to evolve was last summer, when a virus with a mutation in its spike protein became the dominant strain. Since then, multiple new variants have emerged, often appearing in people who had been fighting the infection over a period of months, giving the virus time and opportunity to adapt to and evade the patient’s natural immune response and medical treatments. The virus can shift, like the flu, within a single patient, as it circulates between people, and even as it jumps from humans to other populations.

Early in the pandemic, we suspected that reinfection by SARS-CoV-2 was possible, but now we know it to be true. The latest case of reinfection is a man in France, initially infected in September and reinfected in January with the South African variant of the virus. Perhaps the most troubling part of the story is that though his first infection came and went with relative ease, whatever immunity he developed has done little to protect him against severe disease this second time around — he is now hospitalized and critically ill from the second infection. This real world reinfection adds emphasis to the results that scientists in China had already generated in their labs — SARS-CoV-2 can mutate to reinfect those who have already generated immunity to a previous strain of the virus.

The likelihood of reinfection is exacerbated by the fact that immunity to SARS-CoV-2 fades over time, again like the flu. With influenza, recent studies have shown that immunity often disappears within a year, potentially allowing the same strain of the virus to reinfect the same person after their immunity fades. With SARS-CoV-2, the timeline may differ but the end result is the same. From what we know today, naturally induced immunity from neutralizing antibodies can begin to decline as early as three months after infection. While we suspect that even a declining level of neutralizing antibodies can still afford someone protection against reinfection by the same strain of the virus, it’s unclear how long that protection might last. What we know with certainty is that eventually those antibodies disappear. Protection afforded to us through vaccination may last longer, but it is still unclear for how long.

With all the similarities between influenza and coronaviruses and how our bodies respond to infections from each, we should look at the decline in infections today in the same way we look at the decline in new flu infections which has also been dropping since the start of this year. Enough people may have been infected with the current dominant strain that we are now achieving some degree of seasonal immunity. But the curve of new infections will tilt upwards again once immunity fades or a new strain of the virus takes hold and begins to reinfect widely.

This means we must act now. We likely have only a short window of time before infections begin to rise again. If we double down on current public health efforts — mask-wearing and social distancing — and reinforce them with additional tools, such as widespread testing, contract tracing and mandatory isolation, we may be able to bring new infections down to near zero. This would be an enormous achievement and, maintained over time, would allow us to track and trace new infections, preventing a new outbreak the size of what we saw this past year from ever occurring again.

But there are two outlying conditions that could throw us for a loop. First, the variants. The ones we know of — the UK variant, the South African variant, the Brazil variant, and our own homegrown variants — are already proving far more transmissible than previous strains. Some are also more lethal, some more effectively evade our immune responses, and some are proving resistant to some of our vaccines. Recent reports suggest that some of these variants have even recombined, forming a newer heavily mutated virus. This, in itself, is bad news. But the news could get much, much worse. Lab studies suggest the virus could become up to 640 times more transmissible than current strains. If a highly transmissible variant took hold in the US, all bets on bringing infections down to near zero would be off. If that variant was also more lethal — remember SARS-CoV-1 killed 50% of those aged 65 and older and MERS killed one out of three infected — the results would be catastrophic.

The second outlier is, oddly enough, ourselves. To date, Americans have not proven willing to make the sacrifices required to come close to near zero infections. We’ve refused to wear masks, refused to quarantine, and generally questioned any attempt to restrict individual liberty in the name of public health. I appreciate the debate, but every delay in our willingness to act comes at the expense of American lives. There is no time to lose — we tried last April to flatten the curve, and we failed. We tried again in July when new cases surged again, and we failed again. This may be our last chance.