Abstract

We estimated the effectiveness and cost-effectiveness of changes in concurrent sexual partnerships in reducing the spread of HIV in sub-Saharan Africa. Using data from Swaziland, Tanzania, Uganda and Zambia, we estimated country-specific concurrency behaviour from sexual behaviour survey data on the number of partners in the past 12 months, and we developed a network model to compare the impact of three behaviour changes on the HIV epidemic: (1) changes in concurrent partnership patterns to strict monogamy; (2) partnership reduction among those with the greatest number of partners; and (3) partnership reduction among all individuals. We estimated the number of new HIV infections over 10 years and the cost per infection averted. Given our assumptions and model structure, we find that reducing concurrency among high-risk individuals averts the most infections and increasing monogamy the least (11.7% versus 8.7% reduction in new infections, on average, for a 10% reduction in concurrent partnerships). A campaign that costs US$1 per person annually is likely cost-saving if it reduces concurrency by 9% on average, given our baseline estimates of concurrency. In sensitivity analysis, the rank ordering of behaviour change scenarios was unaffected by potential over-estimation of concurrency, though the number of infections averted decreased and the cost per HIV infection averted increased. Concurrency reduction programmes may be effective and cost-effective in reducing HIV incidence in sub-Saharan Africa if they can achieve even modest impacts at similar costs to past mass media campaigns in the region. Reduced concurrency among high-risk individuals appears to be most effective in reducing HIV incidence, but concurrency reduction in other risk groups may yield nearly as much benefit.

BACKGROUND:

The prime-boost HIV vaccine regimen used in the recent RV144 trial resulted in modest efficacy of 31% over 3.5 years, but was substantially higher in the first year post-vaccination. We sought to explore the potential impact of a vaccine with rapidly waning efficacy in a South African population.

METHODS:

We explored two strategies using a dynamic compartmental epidemic model for heterosexual transmission of HIV: [1] vaccination of a single cohort (30%, 60% or 90% of the initial population), with exponentially waning efficacy, but booster vaccinations at 5- or 2-year intervals, and [2] continuous vaccination of the unvaccinated population at the same coverage levels (30%, 60% or 90%) but with a constant efficacy vaccine of short duration. We also examined potential changes in post-vaccination condom use.

RESULTS:

The single cohort vaccination strategies did not have a substantial impact on HIV prevalence, although without boosters they still prevented 2-6% of the expected infections at 20 years, depending on the population coverage. The 5-year and 2-year booster strategies prevented 8-24% and 17-45% of the expected infections, respectively. Continuous vaccination to maintain population coverage levels resulted in more substantial reductions in population HIV prevalence and greater numbers of infections prevented: HIV prevalence at 20 years was reduced from 23% to 8-14% and the number of expected infections was decreased by 34-59%, depending on the population coverage level. Moderate changes in post-vaccination condom use did not substantially affect these outcomes.

CONCLUSIONS:

An HIV vaccine with partial efficacy and declining protection similar to the RV144 vaccine could prevent a substantial proportion of HIV infections if booster vaccinations were effective and available. Our estimates of the population impact of vaccination would be improved by further understanding of the duration of protection, the effectiveness of booster vaccination, and whether the vaccine efficacy varies between subpopulations at higher and lower risk of exposure.

The supercommittee's failure to reach an agreement on debt reduction will probably result in unexpected reductions of the U.S. nuclear arsenal. That possibility concerns the defense establishment, but it also presents an opportunity: It might finally be possible to have an honest debate about the role of nuclear weapons in U.S. strategy and the prospect for further arms reductions.

Before moving ahead with this conversation, though, it is critical to review and debunk three misguided ideas about nuclear weapons.

The first is that our nuclear world is safe and stable and that all we need to do now is prevent other nations from acquiring nuclear weapons. Though it is undoubtedly true that the U.S. stockpile is safer than ever, the dangers are far from over. Nuclear terrorism remains a threat. Mistakes are possible, too. In just one example, in August 2007, six nuclear warheads disappeared for two days between North Dakota's Minot and Louisiana's Barksdale Air Force bases.

What's more, unsafe nuclear weapons elsewhere remain a major threat. Tensions between nuclear India and Pakistan, the security of the Pakistani nuclear arsenal and the future of the North Korean nuclear weapons program all suggest that the commitment to making U.S. weapons more reliable and secure will not solve the problem.

The second piece of nuclear mythology is that nuclear disarmament has never taken place and never will. Put slightly differently, it is the idea that nuclear history is proliferation history. But nuclear disarmament is far from unprecedented. South Africa, Ukraine, Belarus and Kazakhstan all disarmed. Many nuclear-capable states chose to pursue security without nuclear weapons because policymakers recognized these weapons would endanger rather than protect them. Sweden went down the nuclear path and then decided against it in the late 1960s.

Germany had a nuclear weapons program during World War II but became a law-abiding, non-nuclear member of the Non-Proliferation Treaty. Japan had two nuclear weapons programs during the war and accumulated a significant quantity of plutonium; since then, its authorities thought about restarting a weapons program four times but decided against it.

In each of those cases, most analysts did not believe that giving up nuclear weapons ambitions was possible. They were wrong, and today we all are glad these countries chose the path they did.

The third misguided concept is that reducing the size of the U.S. nuclear arsenal will lead to proliferation. Those who believe this think that countries that no longer feel protected by U.S. nuclear weapons will start building their own to protect themselves. Although this might have some validity, it should be assessed on a case-by-case basis.

Historically, many of the states that have disarmed or given up their nuclear-weapon ambitions - including every non-nuclear nation outside of NATO - have done so despite the absence of a nuclear-security guarantee.

On the other hand, states determined to get the bomb, such as the United Kingdom and France, have done so despite security guarantees. Finally, this argument assumes that the role of nuclear weapons in future alliances and geopolitical relationships will be as important as it was in the past. This might be true, but it cannot be considered a fact. It is just a bet on the future and a set of policy priorities.

In 2007, "the four horsemen" - Henry Kissinger, Sam Nunn, William Perry and George Shultz - wrote a highly influential opinion piece in the Wall Street Journal arguing that relying on nuclear weapons for the purpose of deterrence has become "increasingly hazardous and decreasingly effective." Coming from former Cold Warriors from both sides of the political aisle, it legitimized the goal of a world without nuclear weapons and challenged the conventional wisdom.

Now policymakers in Washington and candidates on the electoral trail should embrace the issue, and begin a real conversation with the electorate about the role of nuclear weapons in U.S. policy rather than allowing that policy to be driven by inertia or budget cuts.

This paper was prepared for Stanford University’s Global Food Policy and Food Security Symposium Series, hosted by the Center on Food Security and the Environment, and supported by the Bill and Melinda Gates Foundation.

Food policy makers are increasingly faced with the question of how to adapt to climate change. The increased attention on climate adaptation is partly related to the fact that greenhouse gas emissions and climate change show little sign of slowing, partly because of prospects for large sums of money devoted to adaptation, and partly because of well publicized recent weather events that have affected agricultural regions and rattled global food markets. A common and reasonable reaction from the food policy and agricultural community has been to argue that climate variations have always been a challenge to agriculture, and that climate change just makes addressing these variations more important. A logical conclusion from this perspective is to emphasize activities that help build resilience to unpredictable weather events, as well as to focus on the types of weather variables that exhibit a lot of year-to-year variability and cause the bulk of farmers’ concerns in current climate.

However reasonable as a starting point, this perspective is misguided and risks taking a challenging problem and making it even harder. Anthropogenic global warming (AGW) is fundamentally different from the natural variations driven by internal dynamics in the climate system. Indeed, predicting the course of climate change is less like predicting the weather next week than it is like predicting that summer will be warmer than winter. Progress in climate science has shown that the most indelible hallmarks of AGW will be increased occurrence and severity of high temperature and heavy rainfall extremes in all regions, and increased frequency and severity of drought in sub-tropical regions. Changes in the timing and amount of seasonal rainfall also appear likely in some regions, but at a much smaller pace relative to natural variability. In all of these cases, predictions from climate science are most robust at broader spatial scales, with considerable uncertainty in predicting changes for any single country.

Meanwhile, progress in crop science has shown that most crops show fairly rapid declines in productivity as temperatures rise above critical thresholds, with as much as 10 percent yield loss for +1°C of warming in some locations. Both sub-Saharan Africa and South Asia appear particularly prone to productivity losses from climate change, in part because major staples in these regions are often already grown well above their optimum temperature.

Approaches to climate adaptation should recognize these realities, and should not equate anticipating climate changes with the considerably harder task of predicting next year’s weather. Predicting and building resilience to climate variability still remain important goals for agricultural development, but adaptation efforts should balance these activities with those focused more on the specific threats presented by climate change. Heat tolerant crop varieties and strategies to deal with heavy rainfall provide two examples of important needs. Similarly, balance is needed between the local-scale efforts that attract most of adaptation investment currently, and regional and global networks to develop needed technologies. Given the greater certainty of climate changes at broader scales, as well as the positive track record of international networks for crop breeding, investments in these global systems are very likely to deliver substantial adaptation benefits. Finally, given the downward pressures that climate change will exert on smallholder farm productivity in sub-Saharan Africa, and the critical role productivity gains play in catalyzing an escape from poverty, speeding the pace of investment in African agriculture can also be viewed as a good bet for climate adaptation.