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Vaccines are not making COVID strains like ‘Kraken’ stronger. Here’s how the four shots battle variants

January 9, 2023, 12:29 PM UTC
Medical professional fills a needle with COVID vaccine booster
Scientists have dismissed claims that vaccines are making COVID “stronger.”
Justin Sullivan—Getty Images

Scientists are keen to shut down the idea that vaccines are making COVID strains “stronger” and that those who have had a booster are more susceptible to the disease.

The World Health Organization has confirmed that XBB.1.5—dubbed “Kraken” by biology professor Ryan Gregory—is the most transmissible strain yet.

The Centers for Disease Control and Prevention (CDC) said Friday that the mutation is predicted to make up 28% of cases this week, making it the second most prevalent strain in the U.S., having been responsible for 18% last week.

According to the European Centre for Disease Prevention and Control (ECDC), Kraken is estimated to double the number of people it sickens every nine days, prompting calls for “urgent” development of treatment and vaccines against it.

However, claims that vaccines and boosters are helping the virus get “stronger” have been slammed as “absolute nonsense” by health protection scientists.

The Wall Street Journal published an article posing the question: Are Vaccines Fueling New Covid Variants? given the spread of the virus in highly vaccinated areas such as Northeastern America.

And although XBB.1.5 is immune-evasive to some extent, this is a “normal process of virus evolution” explained Dr. Christopher Chiu of the Imperial Network for Vaccine Research.

No evidence that vaccines have made COVID strains more powerful

Speaking to Fortune, the infectious diseases physician and immunologist added there is no evidence to suggest that vaccines have made COVID strains more powerful or that those who have been jabbed are any more likely to get a severe case of infection.

He was echoed by Roby Bhattacharyya, an assistant professor of medicine at Massachusetts General Hospital, who told Bloomberg a study he led last year showed that vaccination probably didn’t accelerate the initial Omicron wave.

New data last week also showed XBB.1.5 shouldn’t evade prior vaccines and infections any better than a variant already in circulation, as Kraken is slightly less immune evasive than its parent mutation.

Maria Van Kerkhove, technical lead for COVID-19 response at the WHO, added Wednesday that it is unknown if the variant is contributing to a rise in hospitalizations in the Northeast U.S.

Professor Paul Hunter, of the National Institute for Health Research Health Protection Research Unit (HPRU) in Emergency Preparedness, added there is no clear evidence to suggest just how immune-resistant Kraken is.

He highlighted that hybrid immunity—protection for patients arising out of both vaccines and previous infection—was the best defense against systemic or severe infection, leading to hospitalizations and potential deaths.

How current vaccines battle COVID variants

There are currently four main vaccines available to people in the U.S.: Pfizer-BioNTech, Moderna, Novavax, and Johnson & Johnson.


Pfizer’s new bivalent booster is designed to battle both the original SARS CoV-2 virus and the Omicron subvariants BA.4 and BA.5.

Yale Medicine said experts are still establishing how effective the bivalent jab is against more recent mutations such as Kraken, but signposted that a clinical trial in November 2022 by Pfizer showed that levels of neutralizing antibodies in the bivalent booster were around four times higher in people 55 and older than in people of similar ages who got a monovalent booster.


In November, Moderna said its updated COVID-19 booster generated “significantly higher” neutralizing antibodies against the Omicron BA.4/BA.5 subvariants than its earlier shot, with all participants having a 15-fold increase in Omicron BA.4/BA.5 antibody levels from pre-booster levels.


WHO confirmed a Phase III study conducted in the U.S. and Mexico during a period in which multiple variants (Alpha, Beta, and Delta) were in circulation found the Novavax vaccine efficacy against mild, moderate, or severe COVID-19 was 90%.

The company is also developing a COVID-19-Influenza Combination (CIC), with CEO Stanley C. Erck saying, “We believe that like influenza, COVID-19 will also be seasonal moving forward, and that there is room in the market for new alternatives to provide better protection against the impact of influenza, particularly in older adults, and to explore the potential to combine this with protection from COVID.”

Johnson & Johnson

In 2021 data from the Sisonke 2 study confirm that the Johnson & Johnson COVID-19 booster shot provides 85% effectiveness against hospitalization in areas where the Omicron variant was dominant.

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