*Disclaimer: Just like the virus is evolving, so is the science in trying to understand it. Using reputable sources (e.g., scientific research papers, scientific studies, reports from the CDC, Harvard/MIT/Yale Medicine, etc.) versus solely relying on mainstream media is of utmost importance.


The goal of a vaccine is to provide the protection afforded by natural infection, but without risking serious illness or death. The effectiveness depends on a few things: how big an immune response the vaccine induces, how fast any resulting antibodies decay, whether the virus tends to mutate, and the location of the infection. Given the recent surge in COVID breakthrough infections (infections occurring after full vaccination), how effective are the COVID vaccines? Measles shots are good for life; chickenpox ones are good for a couple of decades; and tetanus shots last for at least 10 years. Why are we needing COVID-19 booster shots after only 6 months?


With diseases like tetanus and measles, scientists have identified the threshold of protection, or, the level of immunity sufficient to keep you from getting sick. This gets measured as levels of antibodies per milliliter of blood. By tracking the decline of antibodies in the blood with time, and by also knowing the threshold of protection, scientists can then calculate the durability of protection. With COVID being so new, we simply don’t know this yet. Adding to the challenge is the fact that COVID-19 is mutating, leading to multiple variants. While you can still potentially get sick with COVID while vaccinated (our recent experience), the vaccines do prevent the majority of the vaccinated from hospitalization and death. The majority of hospitalizations and deaths associated with the variant in recent months are almost entirely in unvaccinated individuals. The CDC has said the risk of infection is eight times higher in the unvaccinated than the vaccinated, and the risk of hospitalization or death is 25 times higher.


Historically, the most effective vaccines have used replicating viruses which elicit long immunity. Measles and chickenpox vaccines use these – but the viruses also hardly mutate, unlike SARS-CoV-2, the virus that causes the disease, COVID-19. Non-replicating vaccines and protein-based vaccines don’t last as long but their effectiveness can be boosted with the addition of an adjuvant – a substance that enhances the magnitude of the body’s immune response to an antigen (a foreign or toxic substance). Tetanus and hepatitis A vaccines use an adjuvant as does the new Novavax COVID-19 vaccine (this one is not yet available in the US). What about the other COVID-19 vaccines? Pfizer and Moderna are messenger RNA (mRNA) vaccines that don’t contain any virus. The mRNA delivers a bit of genetic code to your body’s cells – you can think of it like sending a recipe – to make the coronavirus surface protein, known as a spike protein. The proteins made with the mRNA instructions activate the immune system, teaching it to see the spike protein as foreign and develop antibodies with which to fight it. The Johnson & Johnson and AstraZeneca vaccines use a non-replicating harmless adenovirus from a large family of viruses (some of which cause common colds), that has been engineered to carry the genetic code for the coronavirus spike protein. Once the adenovirus enters your body’s cells, they use that code to make the spike proteins and produce an immune response to also fight the virus.


So what about vaccine efficacy against COVID-19? Let’s compare infection versus severe disease. After 6 months, Pfizer efficacy against infection dropped to 84%, although efficacy against severe disease remained high: 97%. Israel reported the vaccine’s effectiveness to be 90% effective against severe disease but only 39% against infection in its population in late June and early July, prompting them to introduce booster shots. A new study (just out on October 4th) is now showing vaccine effectiveness against Delta variant infections during the first month after full vaccination at 93%, but declining to only 53% effective after 4 months. However, vaccine effectiveness against hospital admission for Delta variant patients is still really high: 93% up to 6 months (https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)02183-8/fulltext)


Moderna has shown that at six months after vaccination, efficacy is 90% against infection and more than 95% against developing a severe case. While both Pfizer and Moderna are considered highly effective, several recent studies have shown Moderna to be more protective. One study published in The New England Journal of Medicine found the Moderna vaccine to be 96.3% effective in preventing symptomatic illness in health care workers compared to 88.8% for Pfizer (https://www.nejm.org/doi/full/10.1056/NEJMoa2106599). Another, from the CDC, found Moderna’s effectiveness against hospitalization held steady over a four-month period. However, Moderna did reports its vaccine to be about two times weaker against Delta than against the original virus.

The Johnson & Johnson vaccine has been found to have 72% overall efficacy and 86% efficacy against moderate and severe disease in the U.S., according to analyses posted by the FDA. A recent study has suggested that the J&J vaccine is less effective against Delta (https://www.biorxiv.org/content/10.1101/2021.07.19.452771v1), prompting creation and hopefully distribution soon of a booster shot.

When it comes to AstraZeneca, as far as the Delta variant, two recent studies showed, respectively, that full vaccination after two doses is 60% effective against symptomatic disease (https://www.gov.uk/government/news/vaccines-highly-effective-against-b-1-617-2-variant-after-2-doses)  and 93% effective against hospitalization (https://www.gov.uk/government/news/vaccines-highly-effective-against-hospitalisation-from-delta-variant).


The bottom line is that full vaccination – from any of the vaccines – will still protect you from severe infection, hospitalization and death. If you’re past 6 months vaccinated, a booster shot will increase your protection against infection. If you haven’t received a booster, given that efficacy against infection is waning, keeping distance and wearing masks, especially indoors, will greatly help in reducing the transmission of the highly contagious Delta variant. Stay safe out there!


photo: “Flu Vaccination Grippe” by Daniel Paquet is licensed under CC BY 2.0