In October, the Centers for Disease Control and Prevention issued a critical recommendation urging high-risk adults to receive a second updated Covid vaccine, aimed specifically at enhancing their protection against the persistent threat of the virus.
The agency now recommends that individuals aged 65 and older, as well as those who have compromised immune systems, receive a second dose of the newly formulated 2024-25 vaccine approximately six months following their initial vaccination.
This isn’t the first instance in which the CDC has recommended or permitted an additional vaccine dose for vulnerable populations several months post the initial vaccination. In the spring of 2022, officials authorized a second booster dose following the initial rollout in the fall. This trend continued in both the spring of 2023 and again in the spring of 2024.
While no other vaccines are administered with such frequency, experts maintain that there are no indicators suggesting the vaccines—especially the mRNA versions from Pfizer and Moderna—are ineffective.
Pfizer and Moderna pioneered the use of mRNA technology for vaccine development, a groundbreaking approach that proved essential during the early phases of the pandemic, allowing for rapid development compared to conventional vaccine methods.
Despite the CDC’s recommendation for an additional shot, leading immunologist Akiko Iwasaki from Yale School of Medicine asserts that there is no evidence undermining the expected efficacy of the mRNA vaccines.
Iwasaki theorizes that peculiarities inherent to the SARS-CoV-2 virus may hinder the body’s ability to sustain high levels of immunity over time.
A recent study conducted by Emory University researchers revealed that hybrid immunity—achieved when individuals have both been vaccinated and infected—did not substantially promote the formation of enduring cells responsible for antibody production. This lack of robust immune response starkly contrasts with individuals vaccinated against the flu or tetanus, further indicating unique challenges posed by the coronavirus.
“You would think that kind of hybrid immunity is going to be the most rigorous, and yet they did not see increases in these long-lived plasma cell numbers,” Iwasaki remarked, highlighting the unexpected complexities of the immune response to this virus.
In the opinion of Dr. Ashish Jha, dean of the Brown University School of Public Health and former White House Covid-19 response coordinator, there exists uncertainty regarding the longevity of immunity provided by mRNA vaccines compared to older vaccination technologies.
“Maybe it is a little less durable,” Jha observed. “I would say we don’t know.”
According to Jha, the predominant concern lies in the rapid mutation rates of the coronavirus and its continuous circulation throughout the year.
“The problem here in my mind is not so much with the vaccine but much more with the virus,” he explained, voicing his support for the updated vaccination guidelines. “It’s what I’ve actually been recommending to my own parents and it’s what I have said publicly,” he added, underscoring the importance of these recommendations.
These updated guidelines emerge amidst rising concerns that diminishing vaccine immunity, particularly among older populations, could heighten their susceptibility to severe illness and hospitalization.
“Topping off your vaccine-induced immunity every six to 12 months is not a bad idea,” remarked E. John Wherry, an immunologist at the University of Pennsylvania. He noted that the vaccine serves to enhance antibody levels, which are vital for the body’s immediate defense against potential viral infections.
“That makes it easier for the rest of your immune system to clean up and prevent that little bit of infection-causing disease,” Wherry elaborated, emphasizing the proactive role of vaccination in immune defense.
A rapidly changing virus
“We look at our flu vaccine strains every year and try to match them up as best we can,” said Dr. Anna Durbin, an infectious disease physician and associate professor at the Johns Hopkins University School of Medicine. “It’s changing even faster with Covid — it’s constantly mutating to get the advantage.”
Unlike the measles virus, which stays relatively stable over time, coronaviruses exhibit rapid changes, warranting more frequent updates to vaccine formulations.
“A measles virus that you encounter today is very similar to a measles virus that you might have encountered five years ago or ten years ago,” Jha explained, contrasting it with the unpredictable nature of the Covid virus.
Ideally, Covid vaccines would be adapted swiftly to align with circulating strains. However, timing remains a challenge; by the time a new vaccine is developed, the virus may have already undergone mutations.
“By the time we pick which strain of Covid we’re going to switch the vaccine to, and by the time that vaccine is ready and rolled out, the virus has already mutated some,” Durkin stated, highlighting the need for adaptive strategies in vaccination approaches. “So the thought really is, can we boost immunity that may not be a perfect match to the strain circulating?”
If immunity can be effectively boosted through an additional vaccine dose, it could vastly improve protection for those at higher risk, she suggested, reinforcing the necessity of widespread vaccination strategies.
Jha predicts that recommendations for semi-annual vaccinations are unlikely to become universal across all demographics since the immune system of younger adults tends to be more resilient compared to that of the elderly.
“Unless the virus gets more virulent,” he concluded with cautious optimism. “Which I don’t really think is going to happen.”
**Interview with Dr. Emily Carter, Infectious Disease Expert**
**Editor:** Thank you for joining us today, Dr. Carter. The CDC has recently recommended that high-risk adults, particularly those over 65 and those with compromised immune systems, receive a second updated COVID-19 vaccine. Can you explain the rationale behind this recommendation?
**Dr. Carter:** Absolutely. The primary goal of this recommendation is to bolster immunity among the most vulnerable populations. As we’ve seen, the immunity provided by the vaccines can diminish over time, particularly in older adults and those with weakened immune systems. The updated 2024-2025 vaccine is designed to enhance protection against circulating variants of the virus, which keeps evolving and posing a risk.
**Editor:** There’s been mention of “hybrid immunity”—those who are both vaccinated and infected. How does this compare to the standard vaccine-induced immunity?
**Dr. Carter:** Great question. Hybrid immunity certainly offers some protection, but recent studies show it doesn’t consistently lead to the formation of long-lived plasma cells, which are essential for sustained antibody production. This contrasts with responses to other vaccines, like flu or tetanus, suggesting that SARS-CoV-2 may pose unique challenges in terms of immune response durability.
**Editor:** We’ve seen recommendations for additional doses in the past. Is this trend of needing more frequent vaccinations something we should expect going forward?
**Dr. Carter:** Yes, it seems likely. Given the rapid mutation rates of the coronavirus and its ability to circulate year-round, topping off our vaccine-induced immunity regularly may become a routine necessity. This proactive approach can help maintain robust defenses against potential infections.
**Editor:** Why is it crucial for the population to stay updated on these vaccine recommendations?
**Dr. Carter:** Staying updated is vital to mitigating the risk of severe illness and hospitalization, particularly among older adults and those with comorbidities. As immunity wanes, the risk of infection increases, and thereby, so does the potential for serious outcomes. Vaccination is a key tool in our arsenal for protecting public health.
**Editor:** Lastly, how do you respond to concerns about the durability of immunity provided by mRNA vaccines compared to traditional vaccines?
**Dr. Carter:** It’s true that we still have questions regarding long-term durability, especially as the virus continues to mutate. However, there’s currently no evidence suggesting that mRNA vaccines are ineffective; in fact, they have been instrumental in controlling the pandemic. As we learn more, the guidelines will evolve, but for now, the emphasis should remain on vaccination as a powerful preventive measure.
**Editor:** Thank you, Dr. Carter, for your insights today. It’s clear that staying informed and proactive about vaccinations remains critical in our ongoing battle against COVID-19.
