Key Insights

• The mRNA vaccines (Pfizer and Moderna) are highly effective at preventing severe illness and hospitalization due to the Delta variant.
• These vaccines show reduced effectiveness in preventing infections compared to severe disease, with varying results.
• The Johnson & Johnson vaccine is less effective against Delta infections, but it still provides strong protection against severe illness.
• Breakthrough infections are more frequent with Delta compared to earlier variants.
• While data on the transmission risk from vaccinated individuals is still lacking, it is likely that vaccinated people can contract and spread the Delta variant.
• Immunity from previous infections (prior to Delta) offers some protection against severe disease, but may not adequately prevent infection or transmission.

What Does Vaccine Effectiveness Mean?

Understanding vaccine effectiveness is crucial as it varies depending on context. Essentially, it gauges the reduced risk when comparing vaccinated individuals to those unvaccinated. However, it’s important to specify the type of risk being discussed—infection, disease severity, hospitalization, or mortality.

Infections indicate whether the virus has established itself in the body, while disease refers to the symptoms resulting from that infection. All vaccines reduce disease risk, but not all prevent infection. During a pandemic, both aspects are significant, as infected individuals can transmit the virus, even without symptoms.

General Effectiveness of Covid Vaccines

All three Covid vaccines authorized by the FDA were evaluated primarily on their ability to prevent severe disease, which they do effectively. For instance, if you exposed 100 vaccinated and 100 unvaccinated individuals to the virus, only about five vaccinated individuals would be expected to develop severe disease.

Early research revealed that mRNA vaccines also significantly reduce infection rates, preventing approximately 75% to 90% of infections. This led to the CDC's May guidance that vaccinated individuals could forgo masks, as they were unlikely to spread the virus. However, most of this data emerged while mask mandates were still in place. A subsequent study indicated that the Pfizer vaccine prevented 96% of infections, while Moderna prevented 98%, but these results were also gathered during a period of universal masking.

With the emergence of the Delta variant, which can evade some antibody defenses, the distinction between preventing disease and infection becomes more critical.

How Do Vaccines Fare Against Severe Disease and Hospitalization from Delta?

National health experts, including Anthony Fauci and CDC Director Rachelle Walensky, have consistently affirmed the vaccines' effectiveness against the Delta variant. Recent reports from Israel state that the Pfizer vaccine reduces severe disease risk by 91% and hospitalization by 88%. For every 100 unvaccinated individuals exposed, nine vaccinated individuals would likely experience severe disease.

A July 21 study published in the New England Journal of Medicine focused on the effectiveness of Pfizer and AstraZeneca vaccines in preventing any form of disease. The findings showed that the Pfizer vaccine prevented 94% of alpha variant cases and 88% of Delta cases, while AstraZeneca prevented 75% and 67%, respectively.

While Moderna's data is comparable to Pfizer's, Johnson & Johnson's results, being a single-dose vaccine, may differ. Currently, there’s no indication of increased severe disease in Johnson & Johnson vaccinated individuals.

How Well Do Covid Vaccines Prevent Delta Infections?

In a recent Scottish study, researchers found that the Pfizer vaccine prevented 92% of alpha infections and 79% of Delta infections. The AstraZeneca vaccine showed lower effectiveness, preventing 73% of alpha infections and 60% of Delta infections.

However, Israeli health authorities reported a stark reduction in Pfizer's effectiveness in preventing Delta infections, suggesting it only prevented 39%. This figure has been contested, indicating an underestimation of vaccine effectiveness against infection.

Consequently, it's clear that vaccinated individuals are more likely to experience breakthrough infections with Delta compared to prior variants.

What About Johnson & Johnson and Delta?

Although there are parallels between AstraZeneca and Johnson & Johnson, their efficacy may not be directly comparable due to their differing dosing schedules. Johnson & Johnson's preliminary data indicated some neutralizing activity against Delta, but without specific percentages or real-world case data.

Recent studies suggest the Johnson & Johnson vaccine may be significantly less effective against Delta, with antibody activity shown to be considerably weaker compared to mRNA vaccines. However, there is no current evidence of increased severe disease among those vaccinated with Johnson & Johnson.

Can Vaccinated Individuals Spread Delta?

The pressing question remains: can vaccinated individuals transmit the virus if infected with Delta? The consensus leans toward yes, particularly as the CDC's guidance evolves. Despite earlier assurances, increasing evidence indicates that vaccinated individuals can indeed spread the virus.

Viral load, a key factor in contagion, is significantly lower in vaccinated individuals. However, with Delta's emergence, studies reveal that viral loads in infected vaccinated individuals are now comparable to those in unvaccinated individuals.

Higher viral loads, coupled with reduced infection prevention, indicate that vaccinated individuals may still effectively transmit the Delta variant.

Natural Immunity Against Delta

Some individuals have opted not to vaccinate, believing they possess sufficient immunity from previous infections. However, research shows that antibodies from past infections may be less effective against Delta than those generated by vaccines.

A study assessing antibody strength found that immunity from natural infections was generally weaker against Delta compared to vaccine-induced antibodies. Consequently, individuals with a history of Covid infection remain susceptible to Delta reinfection and can spread the virus to others.