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I took off my face mask when I arrived home from the health centre with my daughter, who had just received her final round of infant vaccines. The outing left me filled with gratitude that I have such ready access to medical resources to protect myself and my family during the COVID-19 pandemic. My experience is far removed from that of so many people who live with the constant threat of infectious disease and who are less insulated from the consequences of a deadly pandemic.

It's in the nature of epidemics that risks and harms are distributed unequally across populations. Health disparities are glaring in the global AIDS epidemic. Disparities in access to effective antiviral treatment, available since the 1990s, explain why there are still 750,000 people dying of AIDS every year.

With the International AIDS Conference starting virtually on 6 July, amidst a global pandemic of another virus, it is more important now than ever that we maintain vital research into saving lives from HIV.

The virus that causes AIDS, HIV is a death sentence for those who don't have access to treatment. Delays or interruptions to treatment, or not adapting it in response to drug resistance can be lethal. Preventing HIV infection rather than relying on treatment is therefore the most equitable public health strategy.

Figuring out how to prevent infection requires understanding how HIV is transmitted. Because it can take weeks, months or even years for someone to learn that they have been infected, it is very tricky to get a handle on what happens when an infection takes place. 

In a , my colleagues and I investigated what happens in the first moments of a person's infection with HIV. We amassed information on over 100 anonymous sexual partners, such as when each partner was infected and the genetic sequences of the viruses in each person. By combining this information in a mathematical model of how HIV changes in a person, we built a 'family tree' of how viruses in both partners were related. This gave us an insight into the first moments of transmission.

We found that, despite infected individuals having millions of virus particles in their body, they are likely to only transmit a single virus particle when they infect someone else. We also looked at why some people get infected with multiple viruses. We expected to find that the route of transmission — that is, whether the infection was between men, or between a man and a woman — affected the number of virus particles transmitted, because previous studies have shown that men are often at a much greater risk of acquiring HIV through a single sexual encounter with another man than with a woman.

However, we found that the route of transmission had little or no effect on the number of transmitted viruses. Instead, the important factor was how long a person had been infected: those people who had recently become infected were more likely to transmit more viruses, as well as more variants of these viruses.

It is known that an infection with more viral variants has a worse prognosis, so the importance of rapid detection in breaking the chain of transmission is magnified by our discovery that there are more variants transmitted in early stage infection.

Even in the UK where there has been a recent ramp up of accessible HIV testing, a minority of new infections are detected within the first few months of infection. Our results may now be used to better inform the global benefits of early detection in different demographic groups.

Even after 40 years of research, we are still uncovering insights into HIV that will hopefully, when combined, lead to new advances that can reduce transmission within groups who are most affected by the constant threat of infectious disease. 

Publication

Ch. Julián Villabona-Arenas, Matthew Hall, Katrina A. Lythgoe, Stephen G. Gaffney, Roland R. Regoes, Stéphane Hué, Katherine E. Atkins. . Science. DOI: 10.1126/science.aba5443