The fall season may finally bring a needed reprieve from the summer heat, but it also creates the perfect conditions for viruses — like SARS-CoV-2 — to thrive.
When the weather gets colder, people spend more time close together indoors (and some research suggests our frontline immune system responses get worse when it’s cold), which means virus-laden respiratory droplets can spread more easily. It’s likely that come the frigid winter months, Covid hospitalization rates will rise (and we’re already seeing an uptick in hospitalizations). These hospitalizations line up with a few dominant variants sweeping the country.
With other measures like masking or social isolation mostly phased out — and unlikely to return — vaccines remain the most effective defense we have against a new Covid wave. But the US isn’t well prepared for a winter surge on that front.
While 69.5 percent of the US population completed their initial series of Covid-19 shots, only 17 percent received the bivalent booster, meant to protect against newer variants, that was rolled out last year.
On Tuesday, a Centers for Disease Control and Prevention (CDC) committee recommended everyone 6 months or older receive the newest Covid shot, which is specifically designed to target prominent variants of the omicron strain. CDC Director Mandy Cohen signed off on the committee’s recommendation later that evening. This means the newest Covid shots could be available at pharmacies later this week.
The recommendation that everyone should get the Covid booster, regardless of their prior vaccination status, prompts a question: Should we start expecting yearly reformulations like we see with the yearly flu shot? And is there any way to boost the number of people who will get a booster?
“We should start viewing this as just this annual booster that we’re going to need, very much like the flu vaccine itself,” Thomas Duszynski, an epidemiologist with Indiana University’s Fairbanks School of Public Health, said of Covid shots. “We need those vaccines to protect us.”
Despite the CDC’s recent recommendations, not all experts believe a booster will be necessary for everyone every year, particularly young healthy people who have been previously fully vaccinated. In the years to come, the path forward for Covid shots isn’t as clear as the flu vaccines, which are recommended for pretty much everyone, every year, regardless of their prior vaccination history.
Let’s break it down.
The new boosters, explained
Updated Covid-19 boosters are needed for two fundamental reasons: first, that the virus is continually evolving, and second, that our immunity wanes over time.
Since the pandemic first began in 2020, Covid-19 has evolved and mutated into over three dozen various strains and variants. (While, colloquially, strain and variant are often used interchangeably, a variant — such as Eris — refers to a form of a virus that differs only slightly from the original form, while a strain — such as omicron — has distinctly different properties that make it more easily distinguishable from the original virus.) These strains and variants sometimes possess characteristics that can help them evade our weakening immunity, and therefore reproduce better. In turn, scientists are reformulating Covid-19 boosters designed to try to keep up with these mutations — pitting science against viral evolution.
The original Covid vaccines, which became available in the winter and spring of 2021, were monovalent shots — that is, immunizations directed at only one disease strain. The newer bivalent shots that arrived in 2022 protect against the original strain of Covid as well as omicron subvariants.
The newest vaccine will protect against specific omicron subvariants. In theory, these updated vaccines help immune systems evade the most infectious variants of the moment, and can be used as a first-line Covid vaccine for those who never received the initial immunizations.
The latest data from the CDC shows a 8.7 percent spike in Covid hospitalizations between August 26 and September 2. Many of these cases can probably be credited to the omicron subvariant, EG.5, (part of the branch targeted by the new booster and informally named Eris), which causes an estimated 21.5 percent of current Covid-19 cases. A new variant (with a significant number of mutations), BA.2.86, nicknamed Pirola, also just popped onto public health officials’ radars. As Vox’s Keren Landman reports, the data on Pirola suggests it’s unlikely to lead to a substantial Covid surge on its own.
Vaccines, like natural infections, prompt the body to produce virus-fighting antibodies. These proteins are good at fighting infection, but they don’t stay in our bodies forever. “Your protection is the best in the first three months that you’ve gotten the vaccination or infection,” said Taison Bell, an associate professor of medicine at the University of Virginia Health.
During that time, you have a lot of new antibodies that can prevent infections from settling in. After a time, the body naturally breaks down antibodies, like it does all proteins. “It’s antibodies that really prevent mild infection,” said Paul Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “But antibodies are short-lived.”
After a few months, antibodies begin to wane. But T-cells and B-cells — other immune system cells that are boosted by vaccination — have long memories, and while they don’t prevent all infections, they do still protect against severe disease.
The bottom line is that while even reformulated vaccines may be limited in their ability to protect against infection — especially after the first few months — they remain highly effective at preventing hospitalization and death (due to these memory cells).
The case for thinking of Covid boosters like an annual flu shot
At one time, the world hoped Covid would need a one-off vaccine, but with its ever-evolving nature, it’s become clear that’s not the case. The virus will likely require regularly updated formulations of the vaccine to keep up our defenses and protect vulnerable populations.
If this sounds familiar, it’s because we have long faced a similar situation with seasonal influenza, the flu.
The flu constantly evolves, which is why every year we need a vaccine to fight off whatever strain of the virus is working its way across the globe. Like Covid-19, the flu is particularly dangerous for older folks. It can cause fever, headache, sore throat, and muscle aches, and results in 3 to 5 million severe cases a year.
Both Covid and the flu can now be called endemic diseases, said Duszynski, which means new cases will always occur. This means it’s likely the “culture around Covid-19 shots will become very similar to that around flu shots,” meaning wherever you’re asked if you want a flu vaccine — at a pharmacy or a student health center, for example — you’ll also be asked if you want a Covid shot, he added. In the 2022 flu season over half of American children received a flu shot and nearly half of US adults received one (remember, only 17 percent of people received a Covid booster the last time they were rolled out).
Despite these similarities, as the US is experiencing now, Covid’s seasonality is far different from that of something like the annual flu. While the flu very consistently peaks in winter, Covid outbreaks seem to occur all year, said Rebecca Wurtz, an infectious disease physician and a professor of health policy and management at the University of Minnesota School of Public Health.
“The difference between flu and Covid so far has been that Covid-19 was able to cause waves in between the winter season, or after and before the winter season,” said Bell. “I think what we’re settling into is that we’ll have a pattern where we have a fall updated booster. And that’s a little bit easier for people to keep in mind. Once the weather gets a little bit cooler out, that’s when it’s time to get both our flu vaccine and the Covid booster. They’re different viruses … But I think just for the purpose of giving people a routine, having a yearly update is going to make sense going forward.”
It’s possible that Covid shots will need to be delivered bi-annually (in the summer and winter) to correspond with historical waves in cases.
Covid has yet to settle into a consistent seasonal pattern, as Landman reports. So, the risk of Covid is “year-round,” Wurtz added, meaning revaccination may have to happen more frequently, and with shots that “more accurately reflect what is circulating in the population” when the doses are administered.
No matter the frequency, the goal is to make Covid vaccines as accessible as possible to the groups that need them most. “We want to increase those opportunities to get people vaccinated,” Duszynski said. People are more likely to get vaccinated if shots are inexpensive and easily accessible. “If I have to go out of my way, or I have to pay an extraordinary amount, I am less likely to get vaccinated. So I think that we’re going to see more of those opportunities occur simply as a public health effort to get people vaccinated,” he said.
So we might see more regular updates, making the Covid boosters seem more like an annual flu shot. But there’s a wrinkle here: The CDC might not recommend that everyone get it every year.
For younger, healthier folks who have been previously vaccinated, the picture is murkier
At different points in time, the CDC has recommended different groups of people get Covid booster shots.
In September 2022, the CDC recommended everyone 12 and up receive a bivalent Covid-19 booster (which protects against the original and omicron strains of SARS-CoV-2). Months later, in April 2023, these recommendations were updated: if you hadn’t received a Covid shot, then you could use the booster as your one and only vaccine. But notably: unless part of a high-risk group, the CDC advised people not to receive multiple bivalent boosters, if they had already been vaccinated.
Now, this week, the CDC recommends that everyone 6 months and older receive the new monovalent shot (which only protects against XBB variants).
That makes things simple. But still, some experts believe the benefit of such broad recommendations could be minimal.
“Last year, the CDC recommended the vaccine be given to everyone over 6 months of age. I don’t think that was the right recommendation, and I think that what they may do this year is the same thing,” Offit said before the CDC’s recommendations were made Tuesday. “I think the more reasonable recommendation would be to target high-risk groups. The question is who’s being hospitalized with severe disease? And at least for SARS-CoV-2, the answer is certain high-risk groups; people over 75, people who are immunocompromised, people who have multiple health problems, and pregnant people.”
For low-risk people who’ve been vaccinated with any formulation of the vaccine, protection against severe disease doesn’t seem to wane as the virus evolves into new variants (given that their T-cells are still primed to defend against severe infection), said Offit. This means those who are young and healthy should actually be protected from hospitalization and death if they have received even one shot.
Notably, the UK has already taken steps to limit who can receive its most updated Covid shot. In April the country narrowed down its Covid booster eligibility criteria, making only those 75 years and older, residents in care homes, and those over 5 years of age with a weakened immune system eligible for vaccination.
That said, Offit stressed: “I don’t think there is a downside for a healthy young person to receive a booster dose. I think of it as low-risk, low-reward.”
There are also still some unknowns about Covid and its impact on the body. And because of these unknowns, some experts agree with the CDC, and think it is wise for young healthy people who previously got the bivalent booster to be boosted again.
“We’re still going to learn over time the consequences of Covid infection without vaccination or under-vaccination because long Covid is still a thing we’re trying to understand,” said Bell. “I think it’s better to stack the deck in your favor and get vaccinated when you can so that you are in the best shape to both not have severe symptoms, and, particularly if you’re high-risk, stay out of the hospital.”
Ultimately, for young and healthy people, vaccination may prevent even mild cases of the disease for at least a short period of time (which could prevent the little-understood side effects that cause long Covid).
Will the vaccines keep getting updated?
It’s also still difficult for scientists to predict the evolution of Covid, and therefore anticipate the need for updating the boosters yet again.
The flu vaccine, which was first licensed for public use in 1945, is produced on a fairly regimented cycle. Every March the FDA Vaccine Advisory Committee identifies strains that are circulating in parts of the world already experiencing winter — such as Australia — and then rolls out a vaccine targeting those strains in fall. (On a global scale, WHO provides guidance, data, and support in identifying flu strains and developing effective vaccines.)
For Covid, the committee convened in June to predict strains that will likely be circulating in a few months based on those that had been circulating at the moment, said Offit. The problem with that is the most prominent strains even three months ago were very different from the ones circulating now.
At the beginning of June, EG.5 accounted for only 3.4 percent of Covid-19 cases. But today, it’s the most prominent strain, causing 21.5 percent of cases in the country. The second-most dominant variant is FL.1.5.1 (Fornax), which now accounts for 14.5 percent of cases. In June, it accounted for 0.2 percent of cases.
The September booster targets XBB variants (the FDA Vaccine Advisory Committee specifically advised targeting XBB.1.5). While this isn’t a perfect match for what is in circulation now — only 3.1 percent of cases are currently XBB.1.5 — these prominent new variants do stem from XBB. Experts hope and predict the September booster should therefore still be effective.
“I think the hope continues to be the same,” said Maureen Miller, an epidemiologist with the Columbia University Mailman School of Public Health. “That getting the updated vaccines that are more closely related to the variants that are circulating, will prevent hospitalization and death.” The CDC consistently monitors the effectiveness of Covid-19 vaccines to understand the success of current shots and the potential need for updated boosters in the future.
When should you get a booster?
Individuals who want to get boosted now should do so with the newest shot, as it is more targeted toward variants circulating currently.
Planning when to get it really depends on your current needs and the state of the ongoing wave, experts say.
The booster takes seven to 10 days to generate an antibody response, said Bell. So if you know you have upcoming travel or a large gathering to attend, you’ll want to plan to get your vaccine at least that many days before, he added, while also still ensuring those plans fall within the three months of peak immunity.
Planning when you revaccinate will also depend on whether or not there are surges in virus cases. “I think it’s going to require … monitoring the Covid status, kind of like we monitor the weather, and being thoughtful about how we protect and prepare ourselves,” said Wurtz. She added this essentially means “thinking forward,” looking at the next one to four months and deliberating what the “Covid climate” is like. Of course, this may be difficult given that in recent months Covid case tracking has ramped down. Hospitalization rates and wastewater tracking are the best ways now for people to track Covid outbreaks.
While there is still plenty of uncertainty around what a world with Covid will look like in the long term, we do have some clarity on how the virus and our shots against it behave, in part because we already have the experience of the flu.
“I think what will happen is it’ll sort of evolve to what happens with the flu every year, or RSV,” Offit said. “It’ll join the pantheon of winter respiratory viruses that collectively cause hundreds of thousands of hospitalizations and tens of thousands of deaths.”
While these viruses continuously evolve to outmaneuver our immune systems, we are also routinely updating our vaccines to fight them off. And this year, humans may have a leg up. For the first time in history, we have vaccines against all three of these deadly winter viruses — RSV, Covid, and the flu. There’s never been a better time to get vaccinated, if eligible.
Update, September 13, 10 am ET: This story was originally published on September 9 and has been updated to reflect the CDC’s new recommendations on who should receive the newest Covid-19 vaccination.